Apparatus and methods for detecting overlay errors using scatterometry

ABSTRACT

Disclosed are techniques, apparatus, and targets for determining overlay error between two layers of a sample. In one embodiment, a method for determining overlay between a plurality of first structures in a first layer of a sample and a plurality of second structures in a second layer of the sample is disclosed. Targets A, B, C and D that each include a portion of the first and second structures are provided. Target A is designed to have an offset Xa between its first and second structures portions; target B is designed to have an offset Xb between its first and second structures portions; target C is designed to have an offset Xc between its first and second structures portions; and target D is designed to have an offset Xd between its first and second structures portions. Each of the offsets Xa, Xb, Xc and Xd is preferably different from zero; Xa is an opposite sign and differ from Xb; and Xc is an opposite sign and differs from Xd. The targets A, B, C and D are illuminated with electromagnetic radiation to obtain spectra S A , S B , S C , and S D  from targets A, B, C, and D, respectively. Any overlay error between the first structures and the second structures is then determined using a linear approximation based on the obtained spectra S A , S B , S C , and S D .

CROSS REFERENCE TO RELATED PATENT APPLICATION

This application claims priority of the following co-pending U.S.Provisional Patent Applications: (1) application Ser. No. 60/431,314,entitled METHOD FOR DETERMINING OVERLAY ERROR BY COMPARISON BETWEENSCATTEROMETRY SIGNALS FROM MULTIPLE OVERLAY MEASUREMENT TARGETS, byWalter D. Mieher et al., filed 5 Dec. 2002, (2) application Ser. No.60/440,970, entitled METHOD FOR DETERMINING OVERLAY ERROR BY COMPARISONBETWEEN SCATTEROMETRY SIGNALS FROM MULTIPLE OVERLAY MEASUREMENT TARGETSWITH SPECTROSCOPIC IMAGING OR SPECTROSCOPIC SCANNING, by Walter D.Mieher, filed 17 Jan. 2003, (3) application Ser. No. 60/504,093,entitled APPARATUS AND METHODS FOR DETECTING OVERLAY ERRORS USINGSCATTEROMETRY, by Walter D. Mieher, filed 19 Sep. 2003, (4) applicationSer. No. 60/449,496, entitled METHOD AND SYSTEM FOR DETERMINING OVERLAYERRORS BASED ON SCATTEROMETRY SIGNALS ACQUIRED FROM MULTIPLE OVERLAYMEASUREMENT PATTERNS, by Walter D. Mieher, filed 22 Feb. 2003, and (5)application Ser. No. 60/498,524, filed 27 Aug. 2003, entitled “METHODAND APPARATUS COMBINING IMAGING AND SCATTEROMETRY FOR OVERLAYMETROLOGY”, by Mike Adel. These applications are herein incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to determination of overlay betweenstructures formed in single or multiple layers. More particularly, itrelates to determining overlay based on diffraction of radiationinteracting with such structures.

In various manufacturing and production environments, there is a need tocontrol alignment between various layers of samples, or withinparticular layers of such samples. For example, in the semiconductormanufacturing industry, electronic devices may be produced byfabricating a series of layers on a substrate, some or all of the layersincluding various structures. The relative position of such structuresboth within particular layers and with respect to structures in otherlayers is relevant or even critical to the performance of completedelectronic devices.

The relative position of structures within such a sample is sometimescalled overlay. Various technology and processes for measuring overlayhave been developed and employed with varying degrees of success. Morerecently, various efforts have been focused on utilizing radiationscatterometry as a basis for overlay metrology.

Certain existing approaches to determining overlay from scatterometrymeasurements concentrate on comparison of the measured spectra tocalculated theoretical spectra based on model shape profiles, overlay,and film stack, and material optical properties (n,k dispersion curves),or comparison to a reference signal from a calibration wafer.

Existing approaches have several associated disadvantages. For example,a relatively large number of parameters must be included in the profile,overlay, and film modeling to accurately determine the overlay. Forexample, in some approaches using simple trapezoidal models for both theupper and lower layer profiles, the minimum number of pattern parametersthat must be included is seven, including overlay. If film thicknessesvariation is included in the model, the number of parameters increasescorrespondingly. A large number of parameters could require increasedprocessing resources, may introduce corresponding errors, and may delaythe results, thereby possibly decreasing throughput and increasinginefficiencies and costs. For example, comparison of a measured spectrumto calculated reference spectra takes longer with more parameters,whether a library-based approach is used or a regression approach isused.

Another disadvantage of certain existing approaches to determination ofoverlay based on scatterometry is the detailed knowledge of the filmstack, film materials, and pattern element profiles that may be requiredto determine accurate theoretical spectra to compare to the measuredspectra.

Yet another disadvantage of certain existing approaches to determinationof overlay based on scatterometry is the accurate knowledge of thescatterometry optical system that may be required to determine accuratetheoretical spectra to compare to the measured spectra.

Therefore, in light of the deficiencies of existing approaches todetermination of overlay based on scatterometry, there is a need forimproved systems and methods for determination of overlay based onscatterometry.

SUMMARY OF THE INVENTION

Accordingly, mechanisms are provided for determining overlay errorbetween two layers of a sample. In one embodiment, a method fordetermining overlay between a plurality of first structures in a firstlayer of a sample and a plurality of second structures in a second layerof the sample is disclosed. Targets A, B, C and D that each include aportion of the first and second structures are provided. Target A isdesigned to have an offset Xa between its first and second structuresportions; target B is designed to have an offset Xb between its firstand second structures portions; target C is designed to have an offsetXc between its first and second structures portions; and target D isdesigned to have an offset Xd between its first and second structuresportions. Each of the offsets Xa, Xb, Xc and Xd is preferably differentfrom zero; Xa is an opposite sign and differs from Xb; and Xc is anopposite sign and differs from Xd. The targets A, B, C and D areilluminated with electromagnetic radiation to obtain spectra S_(A),S_(B), S_(C), and S_(D) from targets A, B, C, and D, respectively. Anyoverlay error between the first structures and the second structures isthen determined using a linear approximation based on the obtainedspectra S_(A), S_(B), S_(C), and S_(D).

In general, an error offset E may be determined by analyzing at leastthe measured spectra from four or more targets A, B, C, and D eachhaving offsets between two patterned layers, such as offsets Xa throughXd. This analysis can be performed without comparing any of the spectrato a known or reference spectra. In other words, the error determinationtechniques of the present invention do not require a calibrationoperation.

In a specific implementation, determining any overlay error isaccomplished by (i) determining a difference spectrum D1 from thespectra S_(A) and S_(B), (ii) determining a difference spectrum D2 fromthe spectra S_(C) and S_(D), (iii) and determining any overlay error byperforming a linear approximation based on the difference spectra D1 andD2. In a further aspect, the linear approximation is based on a propertyP1 of the difference spectrum D1 and a property P2 of the differencespectrum D2.

In one target implementation, each of the targets A, B, C, and Dcomprises a grating structure Ga1 having periodic structures with aperiod Ta1 disposed at least partially within the first layer and agrating structure Ga2 having periodic structures with a period Ta2disposed at least partially within the second layer. The first periodTa1 and the second period Ta2 are substantially identical, and theoffsets Xa, Xb, Xc, and Xd are each produced by offsetting thestructures with the period Ta1 of the grating structure Gal with respectto the structures with the period Ta2 of the grating structure Ga2 bythe sum of a first distance F and a second distance f0, wherein thesecond distance f0 has a smaller absolute value than the first distanceF.

In another aspect, the invention pertains to an optical system operableto determine overlay error between two layers of a sample. The systemgenerally includes one or more processors configured to perform one ormore of the described method operations.

These and other features and advantages of the present invention will bepresented in more detail in the following specification of the inventionand the accompanying figures which illustrate by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relative distribution of designed overlay offsetsXa, Xb, Xc, and Xd for corresponding interlayer patterns (overlaytargets) A, B, C, and D according to an embodiment of the presentinvention.

FIG. 2( a) is a side view illustration of a patterned top layer L2 beingoffset by an amount +F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( b) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( c) is a side view illustration of a patterned top layer L2 beingoffset by an amount +F+f0 from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( d) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F+f0 from a patterned bottom layer L1 in accordancewith one embodiment of the present invention.

FIG. 2( e) is a side view illustration of a patterned top layer L2 beingoffset by an amount +F+f0+E from a patterned bottom layer L1 inaccordance with one embodiment of the present invention.

FIG. 2( f) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F+f0+E from a patterned bottom layer L1 inaccordance with one embodiment of the present invention.

FIG. 3( a) is a flow diagram illustrating a procedure for determiningoverlay in accordance with one embodiment of the present invention.

FIG. 3( b) shows a graphical representation of an approach todetermination of overlay according to an embodiment of the presentinvention.

FIG. 4 is a diagrammatic representation of a conventional microscopicimaging system.

FIG. 5( a) is diagrammatic representation of a microscopic imagingsystem having a numerical aperture (NA) optimized for scatteringcharacteristics in accordance with a first embodiment of the presentinvention.

FIG. 5( b) is diagrammatic representation of a microscopic imagingsystem having a numerical aperture (NA) optimized for scatteringcharacteristics in accordance with a second embodiment of the presentinvention.

FIG. 5( c) is diagrammatic representation of a microscopic imagingsystem having a numerical aperture (NA) optimized for scatteringcharacteristics in accordance with a third embodiment of the presentinvention.

FIG. 5( d) is diagrammatic representation of a microscopic imagingsystem having a numerical aperture (NA) optimized for scatteringcharacteristics in accordance with a fourth embodiment of the presentinvention.

FIG. 5( e) is a top view representation of an imaging spectrometer,multiple site field-of-view example in accordance with one embodiment ofthe present invention.

FIG. 6 is a diagrammatic representation of a system for selecting one ormore wavelength ranges in accordance with one embodiment of the presentinvention.

FIG. 7 is a diagrammatic representation of a simultaneous, multipleangle of incidence ellipsometer.

FIG. 8 is a schematic view of a spectroscopic scatterometer system inaccordance with one embodiment of the present invention.

FIG. 9( a) shows a plurality of targets placed substantially-collinearlyalong either an X-direction or a Y-direction in accordance with a firstembodiment of the present invention.

FIG. 9( b) shows four targets disposed collinearly along theX-dimension, and four targets disposed collinearly along the Y-dimensionin accordance with a second embodiment of the present invention.

FIG. 10 shows an example incidence line and field of view for use in atechnique for scatterometric overlay determination using an incidenceline in accordance with one embodiment of the present invention.

FIG. 11 a is a top view representation of a first combination imagingand scatterometry target embodiment.

FIG. 11 b is a top view representation of a second combination imagingand scatterometry target embodiment.

FIG. 11 c is a top view representation of a third combination imagingand scatterometry target embodiment.

FIG. 12 is a diagram of a combined mark, in accordance with oneembodiment of the present invention.

FIGS. 13A-13D show variations of a combined metrology tool, inaccordance with several embodiments of the present invention.

FIG. 14 is a flow diagram using a combined metrology tool, in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Reference will now be made in detail to a specific embodiment of theinvention. An example of this embodiment is illustrated in theaccompanying drawings. While the invention will be described inconjunction with this specific embodiment, it will be understood that itis not intended to limit the invention to one embodiment. On thecontrary, it is intended to cover alternatives, modifications, andequivalents as may be included within the spirit and scope of theinvention as defined by the appended claims. In the followingdescription, numerous specific details are set forth in order to providea thorough understanding of the present invention. The present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process operations have not been described indetail in order not to unnecessarily obscure the present invention.

An aspect of the present invention provides a set of four or morescatterometry overlay targets which have been formed on a sample, suchas a semiconductor device. A pattern could also be described as a“pattern or interlayer pattern”, with the two terms being synonymousunder most circumstances. In a particular implementation, the sample hastwo or more layers of a semiconductor device, and the targets areutilized to provide a measure of the placement accuracy of variousstructures comprised in the device. In general, placement accuracy ischaracterized by measurement of an overlay error between two differentlayers of the semiconductor device.

In a specific embodiment, a set of four targets are provided, and eachtarget includes two sets of structures on two different layers which areoffset from each other. In a specific implementation, an offset may bedefined as the sum or the difference of two separate distances: a firstdistance F and a second distance f0, with F being greater than f0.Denoting the four targets as “target A”, “target B”, “target C” and“target D”, the corresponding predetermined offsets for each of thesetargets may be defined as follows for a specific target design:

-   -   Xa=+F+f0 (for target A),    -   Xb=−F+f0 (for target B),    -   Xc=+F−f0 (for target C), and    -   Xd=−F−f0 (for target D).        The offsets for Xa through Xd may be any suitable value for        practicing the techniques of the present invention so as to        determine overlay. For example, Xa and Xb may have different        values of f0 than Xc and Xd.

FIG. 1 illustrates the distribution of offsets Xa, Xb, Xc and Xd alongthe x axis in a particular implementation of the invention. As shown,offsets Xa and Xc are both positive with Xa being larger than Xc. Incontrast, offsets Xb and Xd are both negative with Xd being morenegative than Xb.

The number of targets and the magnitude and sense of their correspondingoffsets may be chosen in any suitable manner so that the techniques ofthe present invention may be practiced to determine overlay error. Aspecific set of targets and their corresponding offsets are describedbelow in relation to FIGS. 2( a) through 2(f). It should be readilyapparent that there are numerous combinations of targets and offsetvalues which may be utilized to practice the techniques and utilize thesystems of the present invention.

FIG. 2( a) is a side view illustration of a patterned top layer L2 beingoffset by an amount F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention. Each layer L1 and L2 ispatterned into a set of structures. A structure may include any suitablefeature, such as a line, trench or a contact. A structure may bedesigned to be similar to a semiconductor device feature. A structuremay also be formed from a combination of different features. Further, astructure may be located on any layer of the sample, e.g., either abovethe top layer of the sample, within any layer of the sample, orpartially or completely within a layer of the sample. In the illustratedembodiment of FIG. 2( a), layer L1 includes the complete structures 204a-c, while layer L2 includes the complete structures 202 a-c.Construction of scatterometry overlay targets structures and methods forproducing them are described in U.S. patent application, havingapplication Ser. No. 09/833,084, filed 10 Apr. 2001, entitled “PERIODICPATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, by Abdulhalim, et al.,which application is herein incorporated by reference in its entirety.

As shown, the structures of the top layer L2 are offset by an amount Ffrom the structures of the bottom layer L1. The structures of the twooffset layers may be located within adjacent layers or have any suitablenumber and types of layers disposed in between the two offset layers.FIG. 2( a) also shows three films T1, T2, and T3 between patternedlayers L1 and L2 and their corresponding structures. To the extent thatany other layers exist between the two layers having the structures,these other layers exhibit at least a minimum degree of transmission forelectromagnetic radiation to permit propagation of the radiation betweenthe layers having the structures.

FIG. 2( b) is a side view illustration of a patterned top layer L2 beingoffset by an amount −F from a patterned bottom layer L1 in accordancewith one embodiment of the present invention. FIG. 2( c) is a side viewillustration of a patterned top layer L2 being offset by an amount +F+f0from a patterned bottom layer L1 in accordance with one embodiment ofthe present invention. FIG. 2( d) is a side view illustration of apatterned top layer L2 being offset by an amount −F+f0 from a patternedbottom layer L1 in accordance with one embodiment of the presentinvention. FIG. 2( e) is a side view illustration of a patterned toplayer L2 being offset by an amount +F+f0+E from a patterned bottom layerL1 in accordance with one embodiment of the present invention. FIG. 2(f) is a side view illustration of a patterned top layer L2 being offsetby an amount −F+f0+E from a patterned bottom layer L1 in accordance withone embodiment of the present invention.

In general, an error offset E may be determined by analyzing at leastthe measured spectra from four or more targets A, B, C, and D eachhaving offsets between two patterned layers, such as offsets Xa throughXd. This analysis is performed without comparing any of the spectra to aknown or reference spectra. In other words, the error determinationtechniques of the present invention do not require a calibrationoperation.

FIG. 3( a) is a flow diagram illustrating a procedure 300 fordetermining overlay in accordance with one embodiment of the presentinvention. In this example, four targets A, B, C, and D are used whichare designed to have offsets Xa through Xd as described above. That is,target A is designed with offset +F+f0; target B with offset −F+f0;target C with offset +F−f0; and target D with offset −F−f0.

Initially, an incident radiation beam is directed towards each of thefour targets A, B, C, and D to measure four spectra S_(A), S_(B), S_(C),and S_(D) from the four targets in operations 302 a through 302 d.Operations 302 a through 302 d may be carried out sequentially orsimultaneously depending on the measurement system's capabilities. Theincident beam may be any suitable form of electromagnetic radiation,such as laser or broadband radiation. Examples of optical systems andmethods for measuring scatterometry signals to determine overlay may befound in (1) U.S. patent application, having patent Ser. No. 09/849,622,filed 4 May 2001, entitled “METHOD AND SYSTEMS FOR LITHOGRAPHY PROCESSCONTROL”, by Lakkapragada, Suresh, et al. and (2) U.S. patentapplication, having application Ser. No. 09/833,084, filed 10 Apr. 2001,entitled “PERIODIC PATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, byAbdulhalim, et al. These applications are herein incorporated byreference in their entirety.

Further embodiments of suitable measurement systems and their use fordetermining overlay error are further described below. In variousembodiments of the present invention, the spectra S_(A), S_(B), S_(C),and S_(D) (and any additional spectra that may be present) could includeany type of spectroscopic ellipsometry or reflectometry signals,including: tan(Ψ), cos(Δ), Rs, Rp, R, α (spectroscopic ellipsometry“alpha” signal), β (spectroscopic ellipsometry “beta” signal),((Rs−Rp)/(Rs+Rp)), etc.

Spectrum S_(B)(−F+f0) is then subtracted from spectrum S_(A)(+F+f0), andspectrum S_(D)(−F−f0) is subtracted from spectrum S_(C)(+F−f0) to formtwo difference spectra D1 and D2 in operations 304 a and 304 b,respectively. Next, a difference spectrum property P1 is obtained fromthe difference spectra D1 and a difference spectrum property P2 isobtained from the difference spectrum D2 in operations 306 a and 306 b,respectively. The difference spectra properties P1 and P2 are generallyobtained from any suitable characteristic of the obtained differencespectra D1 and D2. The difference spectra properties P1 and P2 may alsoeach simply be a point on the each difference spectra D1 or D2 at aparticular wavelength. By way of other examples, difference spectraproperties P1 and P2 may be the result of an integration of averaging ofthe difference signal, equal the an average of the SE alpha signal,equal a weighted average which accounts for instrument sensitivity,noise or signal sensitivity to overlay.

After difference spectra properties P1 and P2 are obtained, the overlayerror E may then be calculated directly from the difference spectraproperties P1 and P2 in operation 308. In one embodiment, a linearapproximation is performed based on the difference spectra properties P1and P2 to determine the overlay error E, while in another technique thedifference spectra properties P1 and P2 are used to approximate a sinewave function or other periodic function which is then used to determinethe overlay error E. One linear regression technique is illustratedbelow with respect to FIG. 3( b). In one example, the overlay result maybe obtained by a statistical calculation (e.g. averaging or weightedaveraging) of overlay results obtained from properties of multiplewavelengths or multiple wavelength ranges.

In a variation of this implementation, if all four targets have the samecharacteristics, such as pitch P, thin film characteristics, structuresize and composition, except for the offsets, and assuming that Xa andXb are opposite in sign and have the same order of magnitude(0.1<Xa/Xb<10), and if 0.05<|Xa/P|<0.45 and 0.05<|Xb/P|<0.45, and if Xais the same sign as Xc and Xb is the same sign as Xd, an estimate of theoverlay error E present between structures within the interlayer targetscan be calculated as follows using a linear approximation based on thedifference spectra properties P1 and P2:E′=((S _(C) −S _(D))*(Xa+Xb)/2+(S _(A) −S _(B))*(Xc+Xd)/2)/((S _(A) −S_(B))−(S _(C) −S _(D)))orE′=(P2*(Xa+Xb)/2+P1(Xc+Xd)/2)/(P1−P2)

where the difference spectra properties P1 and P2 are generally oppositein sign for overlay errors E<f0. If (Xa−Xb)=(Xc−Xd) and E=0, thenP=−1*P2.

Alternatively, if the same values for F and f0 are used for designingeach target offset Xa, Xb, Xc, and Xd, thenE′=(f0*P2+f0*P1)/(P1−P2).

The targets may be used to determine overlay of structures located atleast partially in more than one layer, but could also be used todetermine overlay of structures located substantially in a single layer.

FIG. 3( b) shows a graphical representation of the linear approach fordetermining the overlay error E in accordance with one embodiment of thepresent invention. As shown, the positive portion of the y axis shows achange in the difference spectra property PI as a function of f0+E andthe negative portion of the y axis shows a change in the differencespectra as a function of −f0+E. As described above, the differencespectra properties P1 and P2 are obtained from the difference spectra D1and D2.

The overlay error E may be obtained by analyzing the two points (+f0+E,P1) and (−f0+E, P2). The overlay error E may be determined in oneapproach by performing a linear approximation with the two obtaineddifference spectra properties P1 and P2. Note that there are two pointson the graph where E is zero, while other portions of the graph are afunction of the overlay error E and f0. If the offsets are chosencarefully so as to be in the linear region, then the slope of thepositive portion of the graph (P1/(+f0+E)) should equal the slope of thenegative portion of the graph (P2/(−f0+E). Thus, the overlay error isgiven by E=f0*(P1+P2)/(P1−P2).

According to an implementation of the invention, if there are twotargets with offsets +F and −F that are equal in magnitude but oppositein sign and no other overlay errors, then the 0th diffraction orderscatterometry SE or reflectometry spectra are substantially identicalfrom these two targets (to a good approximation) and the differencesignal between the spectra corresponding to +F and −F is zero. Ofcourse, any property of the difference signal is also zero. If onedeliberately breaks the symmetry (artificially induces an overlay error)by designing an additional offset +f0, then the difference signal D1 isno longer zero and any suitable difference spectra property follows thesame relationship as for an overlay error E. Similarly one can designanother set of overlay targets with an additional offset −f0. Thus, theoverlay error may be determined using a property of the differencesignals D1 (+F+f0, −F+f0) and D2 (+F−f0, −F−f0), and accordingly noseparate calibration step is required.

It should be understood that when the overlay error E is calculated fromthe spectra signals, it may be an estimate of actual overlay error. Thecalculated overlay error E may be denoted as the overlay error (E), oran estimate of the overlay error (E′).

If the pitch between structures is relatively large then the abovedescribed linear approximation techniques generally work well. However,when the pitch is relatively small then additional targets may beproduced on the sample to improve the accuracy of the overlaymeasurements. The number of targets and corresponding scatterometrytechniques which are used depend on the particular materials of thetarget and the scatterometry signal type implemented, among otherfactors. Whether to use four or more targets can be determinedexperimentally or by well known modeling methods. In one embodiment, twoadditional interlayer targets (denoted targets “H” and “J”) are producedon the sample, with corresponding offsets Xh and Xj. Upon beingilluminated by incident radiation, the targets H and J producecorresponding diffracted components, which can serve as a basis fordetermination of an additional difference signal D3 and differencespectra property P3. This property P3 may be analyzed in connection withthe difference spectra properties P1 and P2 to refine the determinationof the overlay E to include non-linear corrections or measurements ofthe errors introduced by using a linear approximation.

One alternative embodiment to the linear approximation methods discussedabove is to treat the scatterometry overlay signal as a periodicfunction and use phase detection methods to determine the overlay error.This embodiment may be preferred in some circumstances, depending onvariables that may include scatterometry overlay target pitch,scatterometry overlay target design, scatterometry overlay (SCOL) targetmaterials, the measured scatterometry signal, and the like.

The overlay error may be extracted from measuring multiple SCOL targetswith preprogrammed additional built-in overlay offsets. (One example ofthe preprogrammed offsets could be Xa, Xb, Xc, and Xd as discussed aboveand in FIG. 1). The number of targets measured may be two, three, four,or more than four, or may vary between different overlay measurementlocations. A scatterometry signal (as a function of the wavelength orincident angle, for example) is acquired from the required SCOL targets.For every arbitrary overlay, this signal is a periodic and even functionof overlay error. A phase detection (or phase retrieval, phaseextraction, or phase determination) algorithm utilizes these propertiesof the signals. The measured signal is represented by a set of evenperiodic functions with a corresponding number of free parameters (oneof these free parameters is the overlay error itself). Different sets ofsuch functions may be used, depending, for example, on the number oftargets measured, scatterometry signal properties, target properties,and information required. The number of targets measured is to begreater or equal to the cumulative number of free unknown parameters.When several (two or more) scatterometry overlay (SCOL) targets (withdifferent pre-programmed offsets) are placed in the immediate vicinityof each other (within 250 microns, for example), the overlay error maybe assumed to be the same for all these targets. Each of the other freeparameters can either vary or not vary from one SCOL target location tothe other one (within the field and/or across the wafer). (Overlay isassumed to vary between different overlay measurement locations).Alternatively, these free parameters (or some of them) may either varyor not vary between X- and Y-SCOL target orientations. Based on theinformation required, the measurement accuracy required and on whethersome free parameters are not varying location-to-location and/or betweenX- and Y-orientations, the total number of SCOL targets per overlaymeasurement location and total number of SCOL targets to be measured perfield and/or per wafer is determined.

An example of a phase algorithm approach to determining overlay errorfrom scatterometry signals from multiple targets is to treat thedependence of the scatterometry signals on overlay error as a periodicfunction. In this case the programmed offsets of the multiple targetsare treated as initial phase offsets and the overlay error is treated asan additional phase. The overlay error can then be determined usingwell-known phase determination or phase retrieval methods. Well knownphase retrieval methods that may include quadrature, 3-bucket, and4-bucket phase retrieval algorithms can be used to determine overlayerror. These phase retrieval methods are listed as examples only and arenot meant to limit the scope of the invention. Phase detection methodsare well known and are commonly used in diverse areas such ascommunications, interferometry, nuclear magnetic resonance, electroniccircuits, to list a few examples. In another embodiment, a combinationof linear, non-linear, and phase retrieval algorithms may be employed todetermine the overlay error.

Certain conditions are preferably met with implementation of the abovedescribed techniques. The measurement areas are substantially identicalin all aspects except for the offsets, e.g., +F+f0, −F+f0, +F−f0, and−F−f0. This is likely accomplished by placing the targets within about100 microns or less of each other and by choosing targets which arerelatively-robust to the process (i.e. they have similar or lesssensitivity to process variation as the device features). In practice,on production wafers, the profiles may deviate from identical fordifferent offsets if the topography from the lower pattern layer(s) andthe upper layer changes in response to interacting with this topography.A difference or error signal between the two targets with differentoffsets is relatively independent of profile variation of the overlaytarget segments and to film variation as long as the profiles are commonto the different targets. This is the substantial equivalent of commonmode rejection of the parts of the signal that are determined by theprofile and the films and the optics. The technique is also preferablyrobust to the range of process variation encountered in a typicalmanufacturing process. The signal differences due to the overlay errorare also preferably larger than the signal differences due to othersources of process variation between the nearby scatterometry overlaytargets (including mask errors).

If in a particular implementation the targets include structures groupedto exhibit the characteristics of lines, then a separate set of targetsmay be required for X and Y overlay measurements. If the overlay targetsare composed of 2-dimensional structures (as seen from a top down view),then it may be possible to use one set of targets to get both X and Yoverlay information. For oblique scatterometry, according to a specificimplementation, it may be advantageous to rotate the orientation of thewafer with respect to the optical scattering plane to measure thedifferent X and Y overlay errors. For true normal incidence, it may bepossible to get both X and Y overlay information from the differentpolarizations without rotating the wafer or the optics.

Cartesian coordinates provide a convenient frame of reference formeasuring overlay within a sample, with the x-y plane being locatedwithin, or substantially parallel with, a layer of the sample, and withthe z axis being substantially perpendicular to the layers of thesample. The Cartesian coordinate system could be fixed with respect tothe sample or could be rotated to reduce the complexity of themeasurements. For example, overlay occurring diagonally across thesample but within a single layer could be described as two-dimensionalx-y overlay in a Cartesian system with the x-y axes substantiallyparallel with the sides of a rectangular sample. That same diagonaloverlay could be measured along a single axis, however, by rotating thex-y axes such that the x axis is parallel with the direction of thediagonal overlay. Three dimensional overlay could be reduced totwo-dimensional overlay by restricting measurements within an x-y planesubstantially parallel with a layer and ignoring any interlayer overlayoccurring in the z direction.

In one embodiment, targets include more than one predefined offset,possibly between different sets of structures located in two layers, orpossibly between different sets of structures located in more than twolayers. In a general case, a target may include an indefinite number oflayers, with all or some of these layers having structures producingpredefined offsets. In a particular implementation, the structures inone or more underlying patterned layers of a target cause changes in theshape or topography of one or more upper layers (disposed above theunderlying patterned layer(s)). In this implementation, the one or moreupper layers may be substantially or partially opaque or absorbing, andat least part of the diffraction signal may arise from the topography ofan upper layer, the topography arising at least in part from theunderlying patterned layer.

According to one embodiment, structures included in a target may beorganized in various configurations and shapes, including, for example,lines, grids, rectangles, squares, curved lines, curved shapes, orcombinations of the foregoing. Such configurations of structures may bedisposed at various locations within the target, and may describevarious angles with respect to the electromagnetic radiation incident onthe target. For example, the sets of structures could be organized as aset of parallel lines perpendicular to the direction of propagation of acollimated set of radiation rays or of a beam incident on the target. Inanother case, the structures organized as a set of parallel lines couldbe disposed at an acute angle with respect to the incident radiation,possibly at an angle of 45 degrees. Such a configuration could beadvantageous by facilitating determination of overlay in both x and ydirections, thereby reducing the need for additional overlay patterns ormeasurements.

1. Scatterometry System Embodiments and Use of Same

Several of the techniques of the present invention may be implementedusing any suitable combination of software and/or hardware system. Forexample, the techniques may be implemented within an overlay metrologytool. Preferably, such metrology tool is integrated with a computersystem which implements many of the operations of this invention. Suchcomposite system preferably includes at least a scatterometry module forobtaining scatterometry signals of the overlay targets, and a processorconfigured to analyze the obtained scatterometry signals to therebydetermine overlay error within such targets. At a minimum, thescatterometry module will usually include (i) a source of illuminationoriented to direct radiation onto a specified location of the sample;and (ii) one or more detectors oriented to detect a scatterometry signalwhich has been scattered by the sample.

At least a portion of the techniques of the present invention may alsobe implemented in an overlay metrology system as an additional overlaymeasurement capability which complements an overlay measurement systemor sub-system based on image analysis such as one used for conventionalbox-in-box or frame-in-frame overlay targets or other imaging typeoverlay measurement structures. Examples of apparatus which combineimaging-based overlay metrology and scatterometry-based overlay aredescribed in the above referenced U.S. Provisional Application No.60/498,524, which is incorporated here by reference. Overlay data fromimaging overlay measurements and scatterometry overlay measurements maybe combined for various uses including: calculating the overlaycorrectables, calculating other overlay corrections, calculating overlayerrors at other locations on the wafer. More use cases for combinationsof imaging overlay metrology and scatterometry overlay metrology arealso described in above referenced U.S. Provisional Application No.60/498,524.

Regardless of the system's configuration, it may employ one or morememories or memory modules configured to store data, programinstructions for the general-purpose inspection operations and/or theinventive techniques described herein. The program instructions maycontrol the operation of an operating system and/or one or moreapplications. The memory or memories may also be configured to storescatterometry data obtained from the targets and overlay error resultsand optionally other overlay measurement data.

Because such information and program instructions may be employed toimplement the systems/methods described herein, embodiments of thepresent invention relates to machine readable media that include programinstructions, state information, etc. for performing various operationsdescribed herein. Examples of machine-readable media include, but arenot limited to, magnetic media such as hard disks, floppy disks, andmagnetic tape; optical media such as CD-ROM disks; magneto-optical mediasuch as floptical disks; and hardware devices that are speciallyconfigured to store and perform program instructions, such as read-onlymemory devices (ROM) and random access memory (RAM). The invention mayalso be embodied in a carrier wave traveling over an appropriate mediumsuch as airwaves, optical lines, electric lines, etc. Examples ofprogram instructions include both machine code, such as produced by acompiler, and files containing higher level code that may be executed bythe computer using an interpreter.

Several of the system embodiments described below are mainly describedand illustrated with respect to a scatterometry module or components forobtaining spectra from four or more targets, while the processor andmemory are not shown.

Imaging Metrology Systems in Which the Numerical Aperture is Optimizedfor Measurement of Scattering Structures

FIG. 4 is a diagrammatic representation of a microscopic imaging system.As shown, the imaging system 400 includes a beam generator 402 forproducing an incident beam 403 of electromagnetic radiation, a beamsplitter 404 for directing the incident beam 405 towards the sample 408.Typically, the incident beam is focused onto the sample by an objectivelens 406. An output beam 409 is then emitted from the sample in responseto the incident beam and passed through the beam splitter 404 throughrelay lens 410 onto imager or camera 412. The camera 412 generates animage of the sample based on the output beam 409.

The system 400 also includes a processor and one or more memory 414which are configured to control the various components, such as the beamgenerator 402, objective lens 406, and camera 412. The processor andmemory are also configured to analyze the detected output beam or imageimplementing the various scatterometry techniques described above.

Traditionally, such imaging systems (such as those used for overlay)have selected numerical apertures (NA's) (e.g., via objective lens 406)to optimize image resolution and to minimize optical aberrations.Selection of NA is typically performed in order to derive the overlayinformation from the variation in intensity over a single target (suchas a box-in-box target) from the geometrical properties of the image.

Conventional imaging systems have relied upon high numerical apertures(NA's), such as 0.7 to 0.9, but doing so results in an expensive opticalsystem which is sensitive to vibration, depth of focus, and opticalaberrations. These issues reduce the achievable precision and cause ameasurement error referred to as “tool induced shift” or TIS.

Scatterometry systems may take measurements at multiple sitessequentially in order to measure both x and y overlay and to eliminateeffects due to variations in other sample parameters, such as filmthickness. This type of measurement process results in significantlyslower operation of the scatterometry tool relative to conventionaloverlay techniques.

In one embodiment of the present invention, the illumination and imagingNA's of an imaging optical system are chosen to optimize the performanceof the instrument on scattering structures by ensuring that only thezero'th diffraction order is collected. One may take advantage of thefact that there exist performance advantages for certain metrology orinspection tasks pertaining to periodic structures when only the zeroorder diffraction is collected by the detection system. Under thiscondition, only the specular reflection is collected. Since the outputwhich is scattered out of the specular is not collected and thenonspecular output may be more sensitive to aberrations, collection ofonly the specular output tends to minimize effects caused by opticsaberrations. This condition also will result in a tool which will beoptimized for relative photometric measurements of multiple sites in thefield of view as described further below. Very low TIS may also beachieved, as compared to conventional imaging systems. Much higherthroughput may also be achieved than with conventional scatterometrysystems.

Choosing the illumination and imaging NA's for a particular imagingsystem is based on the particular configuration of such system. If wenow consider the simplest imaging system in which the numerical apertureNA of illumination and collection are the same and the incident beam innormal to the sample surface, then the condition of “zero orderdiffraction only” can be met if:nλ>2dNA, where n=1.where d is the pitch of the structures of the targets being imaged. Thiscan be restated in terms of the numerical apertures of illuminationNA_(i) and collection NA_(c) of the imaging system as:nλ=d(NA _(i) +NA _(c))This equation indicates that if we are able to constrain the numericalaperture of the illumination system, we can relax the constraint on thenumerical aperture of the collection optics, which may be advantageousunder certain conditions. Thus, the spectral range may be restricted towavelengths greater than twice the product of the pitch and the NA.Under realistic conditions the scattered radiation beam will be wider(more divergent) than the illumination beam. Under realisticcircumstances however, infinitely periodic gratings are not imaged andso the above equations become approximations and the diffracted planewaves become somewhat divergent. So it may be preferable to include amargin of safety in the constraint and require that:nλ≧2dNA(1+ε), where n=1 and ε is small, typically less than 0.5.

As an example, for an NA 0.4 imaging system, wavelengths may berestricted to values greater than 0.8 times the largest pitch, whichdoes not seem to be an unreasonable constraint. For periodic structureshaving features of design rule 70 nm and below, the densest structureswith pitches as low as 200 nm does not constrain the spectral range ofimaging systems with operating wavelengths equal to about 200 nm orlonger, while more isolated features with pitches as large as 500 nm arepreferably measured with wavelengths longer than 400 nm.

It is preferable to account for these constraints when designing animaging spectrometer for metrology and inspection applications. A limiton the spatial resolution of the imaging system is the numericalaperture of the system. It is advantageous to achieve the highestspatial resolution so as to be able to shrink to a minimum the size ofmetrology structures and conserve valuable wafer real estate. Restated,this allows minimization of proximity effects or “crosstalk” betweenadjacent features in the field of view of the imaging spectrometer.Therefore, the highest possible NA is achieved while meeting theconstraint that the zero order diffraction is collected by the detectionsystem.

Another interesting outcome of this constraint is that the highestpossible overlay spatial resolution may be achieved without everresolving the features under test. This may have further advantages asit should ensure that problematic aliasing phenomena are avoided in theimaging system. In a preferred embodiment, an architecture is providedin which the spectral band pass can be easily modified or selected bythe measurement system or algorithm based on the largest pitch in thefeature under test (e.g., as the systems in FIGS. 5A through 5Ddescribed further below). Alternatively, the NA of either illuminationor collection could be easily modified, depending on the largest pitchin the feature under test. Alternatively, all of these embodiments maybe implemented within a single system.

FIGS. 5A through 5E illustrate four embodiments of microscopic imagingsystems having a numerical aperture (NA) optimized for scatteringcharacteristics. As shown in FIG. 5A, the system 500 may have componentswhich operate like the same named components of the system in FIG. 4.The system 500 further includes a wavelength selection device 520 forselecting a particular wavelength. The wavelength selection device 520allows modification of the spectral band by selectively transmitting orselectively reflecting part or portions of the illuminating radiation. Awide variety of well known spectroscopic filtering techniques may beemployed to modify the spectra band, including selecting from a set ofband pass interference filters, continuously varying bandpassinterference filters, grating based spectrometers, Fourier transforminterferometers, acousto-optic tunable filters, to name a few. Thewavelength selection device 520 is positioned within the incident beampath between the beam. The system 500 may also include a polarizercontrol device 522 for causing the incident beam to be in a particularpolarization state and a polarization analyzer 524 for analyzing orseparating out the polar components of the collected beam.

The system of 530 of FIG. 5 b is similar to the system 500 of FIG. 5A,except a wavelength modulation device 532 is used in place of awavelength selection device. The system 540 of FIG. 5C is similar to thesystem 500 of FIG. 5A, except the wavelength selection device 542 ispositioned in the output beam path. The system 550 of FIG. 5D is similarto the system 500 of FIG. 5C, except a wavelength modulation device 552is used in place of a wavelength selection device. The wavelengthmodulation device operates by modulating the intensity of differentwavelengths in different temporal patterns such as different sinusoidalfrequencies. The most common examples of such a device areinterferometers which can be controlled by changing one or more opticalpath lengths in the wavelength modulation device 532 itself (e.g., aninterferometric system, such as in a Michelson, Fabry-Perot, or Sagnacinterferometers). The spectral information may be derived from theresulting signal with a transform analysis like a Fourier transform orHadamard transform, for example.

FIG. 5E is a top view representation of an imaging spectrometer,multiple site field-of-view example in accordance with one embodiment ofthe present invention. In one implementation, spectra from pixels ineach dotted box are averaged to create a spectrum for each of the fourmeasurement targets. Alternatively, spectra from pixels located only ina central region of each dotted box are averaged together. Size andspacing of lines in the illustrated targets are exaggerated foremphasis.

The NA may be selected to ensure that only the zero'th diffraction orderis collected in any suitable manner. In one proposed operationalembodiment:

-   -   1. Two or more sites of differing characteristics are located in        the field of view of the imaging system.    -   2. Images are captured over one or more spectral ranges.    -   3. For each measurement site in the field of view, all or some        of the pixels determined to be within that site are summed or        otherwise combined to characterize the photometric properties of        that site in that spectral range.    -   4. Step 3 is repeated for each spectral range.    -   5. The results for each site over each spectral range are        processed to determine the properties of the sample. For        example, the above described spectral analysis techniques (i.e.,        F+f0) are used on the obtained spectra for each target.    -   6. Steps 1 though 5 are repeated for the plurality of        measurement sites desired across the wafer.

While this example technique describes sequentially capturing imagesover different spectral regions, this could be accomplishedsimultaneously using a system of wavelength dependent beam splitters,filters, and/or mirrors. Alternatively, the same could be affected byusing a device such as a Sagnac interferometer which captures multipleimages at different optical path differences, these being used to deriveinformation equivalent to images taken over different spectral ranges.

Scatterometric Overlay Using Filters

Conventional imaging overlay tools have a high magnification and smallfield of view. Inspection for gross patterning defects is either donemanually on a microscope or automatically on a separate macro inspectiontool. A low magnification overlay tool unfortunately requires multiplesteps or tools, some of which are manual.

In one embodiment, a low magnification microscope with a mechanism forselecting one or more wavelength ranges is provided. This tool alsopreferably uses one or more broadband sources with filters, withmultiple sources covering different wavelength ranges, variable filters,etc. FIG. 6 is a diagrammatic representation of a system 600 forselecting one or more wavelength ranges in accordance with oneembodiment of the present invention. As shown, the system 600 includes abroadband source 602 for generating a multiple wavelength incident lightbeam 604 towards sample 606. A multiple wavelength output beam 608 isscattered from the sample 606 in response to the incident beam 604. Thesystem 600 also includes a filter 610 for selectively passing a portionof the output beam 611 based on wavelength to camera 612. In oneimplementation, the filter is configurable to pass particular colors,such as red, green, blue, or yellow. The camera is operable to generatean image based on the filtered output beam 611.

Measurements of overlay are taken by moving to a location on the samplewhere one or more of the targets in a target set are in the field ofview of the microscope. An image is acquired and the intensity from someor all of the pixels in the image, which includes each individualtarget, are averaged or summed to give an intensity value for the targetat a particular setting of the filter. In one embodiment, the filter isadjusted so as to give a maximum difference between targets. This maysubsequently be normalized with respect to a reference surface, to thenumber of pixels summed, or corrected by a map of the illuminationuniformity within the field of view. The sample or optics may then bemoved until all of the necessary targets in a target set are measured.The overlay value is then determined using the intensity values thusdetermined, as described above, e.g., by:P1=(Ia−Ib) and P2=(Ic−Id)And:Overlay=f0*(P2+P1)/(P2−P1)

This process could be repeated over multiple wavelength ranges toimprove accuracy, precision, and robustness, wherein the wavelengthswhich result in the best contrast are used for the scatterometryanalysis.

Because the magnification is low and the field of view is large comparedto a typical imaging overlay tool and because an image of the area ofthe sample is collected, unlike a conventional reflectometer orellipsometer, analysis of the image may be used to detect other types ofprocessing problems by analyzing the image. For example, if the wrongreticle has been used for one or more processing steps, the image wouldbe materially different. If the resist thickness were incorrect, thebrightness or contrast of the image may be effected. If resist streakingwere present, variation of brightness or contrast over the image may bedetected. In CMP (chemical mechanical polishing) processes, processingerrors such as over-polish, under-polish, etc. could be similarlydetected.

In this embodiment, multiple scatterometry targets can be measuredsimultaneously, increasing the measurement speed. Additionally,processing errors or changes in processing conditions other than overlaycan be detected without the need for a separate inspection tool.

Multi-Angle, Simultaneous Scatterometry

Techniques of obtaining scatterometric measurements may include the2-theta approach, in which scattering intensity from a grating or otherrepeating structure is measured at a plurality of angles by makingmultiple, sequential measurements. Another technique of makingscatterometric measurements is spectroscopic scatterometry. Use of the2-theta approach is very slow, since multiple measurements are typicallymade. Use of spectroscopic scatterometry requires sophisticatedexpensive optics.

In a specific embodiment of the present invention, techniques andapparatus for simultaneous, multi-angle scatterometry are provided.Unlike the 2-theta approach, measurements are made with an apparatuswhich permits scattering intensity to be simultaneously determined formany angles. This technique is far faster than the 2-theta approach.

In order to implement this approach, an optical apparatus such as thatshown in U.S. Pat. No. 5,166,752 by Spanier et al could be used. Thispatent is herein incorporated by reference in its entirety. In thispatent, a multi-angle ellipsometer is shown in, for example, FIGS. 3 and4 of the Spanier et al. patent. FIG. 7 is a diagrammatic representationof a simultaneous, multiple angle of incidence ellipsometer 700. Asshown, the ellipsometer includes a source generator (e.g., components702, 706, 708, 710, and 712) for directing polarized light onto thesurface of sample 714, detection optics (e.g., components 718 through724) for handling and detecting the output beam emitted from the sample,and an analyzer 726 for analyzing the polarization state of the lightreflected from the sample. The source generator includes a light source702, a polarizer 708, a compensator 710 with a variable aperture, and afocusing lens system 712 simultaneously directing polarized light from asingle beam of light from the light source onto the sample's surface atdifferent angles of incidence. The source generator may also include anoptional optical narrow band filter.

The lens system 712 has an effective aperture to focal length ratio forfocusing the light on the sample 714 with angles of incidence which varyover a range of angles of at least one or two degrees. In a particularembodiment, the range of angles of incidence is 30 degrees. Largerangles could be employed for directing rays at the sample 714.

The focusing lens system 712 focuses the polarized light which may befrom a He—Ne laser for example, down to a single small spot or point onthe sample 714. The different incident rays may have widely varyingangles of incidence which are focused on a single, small spot on thesample 714. Thus, the light directed on the small spot on sample 714contains rays at many angles of incidence above and below the angle ofincidence of the central ray through the focusing lens. Each one of theincoming rays is reflected at an angle equal to its angle of incidencewith the polarization state of each of the rays being altered by thatreflection. A detector array 722 is employed to detect a plurality ofrays reflected from the sample 714 individually over different, narrowranges of angles of incidence to simply and quickly obtain data at aplurality of angles of incidence.

The output beam emitted from the sample 714 is directed through outputlens 716, interchangeable aperture 718, polarization analyzer 720, andan optional alternate filter 722 onto detector array 722. The diameter dof the lenses 712 and 716 corresponds to their effective diameter. Inthe illustrated embodiment the lenses 712 and 716 each have a diameter dof 18 mm and a focal length 1 of 34 mm. Other effective lens diametersand focal lengths could be employed so long as a range of angles ofincidence, preferably at least 30 degrees, is provided. The lensdiameter and focal length are chosen with a view toward maximizing thenumber of angles of incidence of the light beams which strike the sample714. In an alternative embodiment, light is transmitted through thesample rather than reflected from a surface of the sample.

The refocusing lens or lenses 712 directs the reflected (transmitted)light toward the detector array 722. However, a refocusing lens need notbe employed as the reflected (transmitted) light could be made todirectly impinge upon an array of detectors. It is preferable that thelenses 712 and 716 do not themselves alter the polarization state of thelight.

The detector array 722 may be a linear, multiple element detectorwherein each of the detector elements can detect a narrow range ofangles of incidence of the rays that illuminate the sample. In thedisclosed embodiment, the array 722 is a solid-state photosensitivedetector array wherein the separate detector elements are all integratedon one circuit chip. Particularly, the detector elements comprise alinear array of photodiodes. While integrated on a single circuit chip,the individual photodiodes can function as separate detectors. Thelinear array of the disclosed embodiment comprises 128 detector elementsarranged in a row to provide data for 128 different angles of incidencewhere the full array is illuminated by the reflected (transmitted)light. The number of individual detector elements could be more or lessthan that in the disclosed embodiment and the detector elements need notbe integrated on a single chip but could be discrete detectors. By usinga plurality of detector elements, it is possible to simultaneouslydetect the light reflected from the surface (or transmitted through thesample) for each of a plurality of different angles of incidence. It isalso possible with the invention to employ a smaller number of detectorelements which could be sequentially moved to mechanically scan thereflected (transmitted) rays for detection but this technique wouldrequire more time and could be less accurate, depending upon positioningaccuracy.

The physical size of each of the detector elements is preferably lessthan the expanse of the reflected rays so that each element detects onlya certain narrow range of angles of incidence on the illuminating side.The output of each of the detectors is used in a conventional manner aswith real time computer techniques (e.g., via analyzer 724) to generatedata in terms of Δ and Ψ for each of those narrow ranges of angles ofincidence. The data is then interpreted in a conventional manner. Itmatters in general which direction the linear array runs; the lineararray preferably runs in the plane of the optical system. In thedisclosed embodiment, the long axis of the linear detector array 722lies in the plane of incidence of the central ray and perpendicular tothe central ray for detecting the maximum number of incidence angles.

Such an ellipsometer could be used to illuminate a scatterometry targetsimultaneously over a range of angles, and an intensity of the scatteredlight is measured over a range of angles simultaneously with an arraydetector or the like. By collecting data from the intensities measuredat those angles, the parameters of the grating or other target can bedetermined. For example, the data can be compared against theoreticalmodels of data derived from techniques such as those mentioned by U.S.Pat. No. 6,590,656, issued Jul. 8, 2003, entitled “SPECTROSCOPICSCATTEROMETER SYSTEM” by Xu et al, which patent is herein incorporatedby reference in its entirety. The data can be compared to theoreticalmodels derived from techniques such as those mentioned by U.S. patentapplication, having application Ser. No. 09/833,084, filed 10 Apr. 2001,entitled “PERIODIC PATTERNS AND TECHNIQUE TO CONTROL MISALIGNMENT”, byAbdulhalim, et al., which application is herein incorporated byreference in its entirety.

The data can be pre-generated and stored in libraries, or generated inreal time during analysis. It is also possible, for techniques likescatterometric overlay, to directly compare measured spectra associatedwith various targets. Such differential measurements can then be used todetermine overlay misregistration.

It would also be possible to perform this technique with a beam profilereflectometer such as that described in U.S. Pat. No. 4,999,014, issued12 Mar. 1991, entitled “METHOD AND APPARATUS FOR MEASURING THICKNESS OFTHIN FILMS” by Gold et al., which patent is incorporated herein byreference in its entirety.

Simultaneous Ellipsometry and Reflectometry

A system for employing a combination of ellipsometers and reflectometersmay be employed to improve the accuracy of scatterometric measurementsof overlay. In one embodiment, two or more ellipsometers are utilized asscatterometers to measure overlay. One of more of these ellipsometerscould be spectroscopic ellipsometers. In another embodiment, two or morereflectometers are utilized as scatterometers to measure overlay. One ofmore of these reflectometers could be polarized reflectometers.Alternatively, a combination of one or more ellipsometers and one ormore reflectometers are utilized as scatterometers to measure overlay.

Measurements can be performed serially (with each tool performingmeasurements at different times), in parallel (with all tools performingmeasurements substantially-simultaneously, or in any other arrangement(e.g., at least two but less than all of the tools performingmeasurements substantially-simultaneously).

In any of the implementations described herein, various tools mayperform measurements at different angles of incidence, includingnormally and obliquely. In a specific implementation, at least two toolsperform scatterometric measurements at substantially the same angle ofincidence but from different directions. For instance, a first toolwould be used for scatterometric measurements in the x direction, and asecond tool would be used for scatterometric measurements in the ydirection. Such a system could eliminate certain common scatteredsignals, with a corresponding increase in accuracy of measurements, andprovide a symmetric configuration.

An advantage of employing a combination of such tools in scatterometricdetermination of overlay is that the accuracy of the measurements couldbe increased. Another advantage in using more than one tool andperforming measurements at more than one angle (or point) of incidenceis to help separate effects affecting the medium of interest (e.g., filmeffects) from overlay. A further advantage is that combinations ofellipsometers and reflectometers already exist in current inspectiontools. Another advantage of employing a combination of scatterometersconfigured to perform scatterometry measurements substantially inparallel on different targets or different target sections could be toreduce the total time required for measurement. Another advantage of aparallel measurement system could be to increase the signal acquisitiontime for each scatterometry overlay target and improve the measurementprecision.

Scatterometric Overlay Determination Using FT processing

A system for scatterometric measurement of overlay using fouriertransform (FT) processing may also be utilized. In one embodiment, aninterferometer is employed to modulate substantially all wavelengths ofa broadband source, and the scattered radiation is detected with a CCDcamera. Substantially all wavelengths of the modulation band arerecorded for each pixel, or for groups of pixels. As the interferometersteps through the modulation band, a spectroscopic image of thescattered signal is produced.

The resulting spectroscopic image may have a relatively large field ofview. For example, the image may include several multiple targets. Thespectroscopic image could be processed on a pixel-by-pixel basis toaccurately determine overlay while eliminating extraneous effects (e.g.,film effects). Alternatively, processing could be performed using groupsof pixels to improve speed and decrease processing resources. Forexample, a group of pixels from each target may be analyzed inaccordance with the above described scatterometry process. In oneimplementation, the images for each corresponding pair of targets issubtracted to obtain difference images D1 and D2. A characteristic, suchas average intensity, of each difference signal is then obtained toresult in P1 and P2, which are then used to determine the overlay error.

In a specific implementation, a Michelson interferometer is used to stepthrough a wavelength modulation band. Alternatively, a Linnikinterferometer, or any other interferometer, could be employed. For eachposition of the mirror, a CCD camera records the scattered signalintercepted in the field of view of the camera. The detected signals maythen be digitized and stored on a pixel-by-pixel basis, or as groups ofpixels. The magnitude of the steps is generally proportional with theaccuracy of the overlay measurement. The speed of the camera (e.g., thenumber of fields per second that the camera can capture) is typicallyproportional with the speed of the measurement. Once the modulation bandis spanned, the signal recorded for each pixel (or group of pixels) maybe used as a basis for a discrete fourier transformation (or DFT). TheDFT provides a spectral profile for each pixel (or group of pixels).This spectral profile for each target may then be used in thescatterometry overlay techniques described in the previous paragraph.Overlay determination can then be performed with increased accuracy.

Multiple Tunable Lasers

A system which has a combination of tunable lasers may be utilized toimprove the accuracy of scatterometric measurements of overlay incombination with measurements performed by various configurations ofellipsometers and reflectometers. The tunable lasers provide radiationincident on the surface of interest. In one embodiment, scatterometricoverlay measurements are performed using targets disposed in at leastone layer of the design under consideration, and the tunable lasersprovide radiation beams incident on the targets.

The measured signals may then be averaged together before or afterprocessing. In one embodiment, measured radiation beams are obtainedfrom targets A, B, C, and D. Two difference signals D1 an D2 from eachpair of targets may then be obtained at multiple tunable laser settings.The signals measured from each target for each tunable laser setting maybe averaged together prior to obtaining the difference signals D1 andD2. Alternatively, each set of differences signals for D1 and D2 may beaveraged together to obtain a single average difference signal D1 andD2. Properties P1 and P2 of the difference signals D1 and D2 (e.g.,integration) may then be obtained. In an alternative embodiment,multiple properties P1 and P2 are obtained for the differentconfigurations of the tunable laser (without averaging the measuredsignals or the difference signals D1 and D2) and the results areaveraged for each signal P1 and P2. The overlay error may then beobtained based on the signals P1 and P2 as described above.

Scatterometric Overlay Determination Using Spatial Filtering

One embodiment expands on the above described embodiment forScatterometric Overlay Determination using FT Processing.

A system for scatterometric measurement of overlay using FT processingin connection with spatial filtering is provided. More particularly, thesignal reflected by at least one scatterometry target is selectivelyfiltered spatially to only process particular signal components.

In the above described embodiment for Scatterometric OverlayDetermination using FT Processing, an interferometer is employed tomodulate substantially all wavelengths of a broadband source, and thescattered radiation is detected with a detector, such as a CCD camera.Substantially all wavelengths may then be recorded for each pixel, orfor groups of pixels. As the interferometer steps through the modulationband, a spectroscopic image of the scattered signal is produced. In thepresent example, where the scattered signal corresponding to a completeimage (or a portion of an image) is collected, only a portion of thesignal corresponding to a single line of pixels is retained.Alternatively, a portion of the signal corresponding to a plurality ofpixel lines, but less than the whole image, is collected. Such aselective collection of the scattered signal can be achieved byspatially filtering the signal to only retain horizontal, vertical oroblique stripes of the signal corresponding to rows of pixels in thedetector or CCD camera. Alternatively, a larger, more complete portionof the scattered signal could be collected at the CCD camera, but theinformation corresponding to undesirable rows of pixels (e.g., an edgeof a target or a border between two targets) may be discarded subsequentto the collection.

The spectroscopic image corresponding to the retained signal may then beprocessed on a pixel-by-pixel basis to accurately determine overlaywhile eliminating extraneous effects (e.g., film effects).Alternatively, processing could be performed using groups of pixels toimprove speed and decrease processing resources. This embodimentprovides higher SNR over conventional processing methods.

In one implementation of the invention, the above described techniquesto determine overlay in reference to the Scatterometric OverlayDetermination using FT Processing embodiment may be used.

Compared to the embodiment for Scatterometric Overlay Determinationusing FT Processing, aspects of the embodiment for ScatterometricOverlay Determination Using Spatial Filtering may improve the processingspeed and the throughput, while decreasing processing resources.

Examples of Spectroscopic Ellipsometers and Spectroscopic Reflectometers

FIG. 8 is a schematic view of a spectroscopic scatterometer system 800,in accordance with one embodiment of the present invention. The system800 combines the features of a spectroscopic ellipsometer 802 andspectroscopic reflectometer 804, each of which may be used for measuringoverlay of a grating structure 806 disposed on a substrate or wafer 808.The grating structure 806, which is shown in a somewhat simplifiedformat in the Figure, may be widely varied. The grating structure 806may, for example, correspond to any of those grating structuresdescribed herein. Both the spectroscopic ellipsometer 802 andspectroscopic reflectometer 804 may utilize a stage 810, which is usedfor moving the substrate 808 in the horizontal xy directions as well asthe vertical z direction. The stage may also rotate or tilt thesubstrate. In operation, the stage 810 moves the substrate 808 so thatthe grating structure 806 can be measured by the spectroscopicellipsometer 802 and/or the spectroscopic reflectometer 804.

The spectroscopic ellipsometer 802 and spectroscopic reflectometer 804also utilize one or more broadband radiation sources 812. By way ofexample, the light source 812 may supply electromagnetic radiationhaving wavelengths in the range of at least 230 to 800 nm. Examples ofbroadband light sources include deuterium discharge lamps, xenon arclamps, tungsten filament lamps, quartz halogen lamps. Alternatively, oneor more laser radiation sources may be used instead of or in combinationwith the broadband light source.

In the spectroscopic reflectometer 804, a lens 814 collects and directsradiation from source 812 to a beam splitter 816, which reflects part ofthe incoming beam towards the focus lens 818, which focuses theradiation onto the substrate 808 in the vicinity of the gratingstructure 806. The light reflected by the substrate 808 is collected bythe lens 818, passes through the beam splitter 816 to a spectrometer820.

The spectral components are detected and signals representing suchcomponents are supplied to the computer 822, which computes the overlayin a manner described above.

In the spectroscopic ellipsometer 802, the light source 812 supplieslight through a fiber optic cable 824, which randomizes the polarizationand creates a uniform light source for illuminating the substrate 808.Upon emerging from the fiber 824, the radiation passes through anoptical illuminator 826 that may include a slit aperture and a focuslens (not shown). The light emerging from the illuminator 826 ispolarized by a polarizer 828 to produce a polarized sampling beam 830illuminating the substrate 808. The radiation emerging from the samplingbeam 830 reflects off of the substrate 808 and passes through ananalyzer 832 to a spectrometer 834. The spectral components of thereflected radiation are detected and signals representing suchcomponents are supplied to the computer 822, which computes the overlayin a manner described above.

In the spectroscopic ellipsometer 802, either the polarizer 828 or theanalyzer 832 or both may include a waveplate, also known as compensatoror retarder (not shown). The waveplate changes the relative phasebetween two polarizations so as to change linearly polarized light toelliptically polarized light or vice versa.

In order to collect more information about the interaction of theincident polarized light 830 with the sample, it is desirable tomodulate the polarization state of the light or modulate thepolarization sensitivity of the analyzer or both. Typically this is doneby rotating an optical element within the polarizer and/or analyzer. Apolarizing element within the polarizer or analyzer may be rotated, or,if at least one of those assemblies contains a waveplate, the waveplatemay be rotated. The rotation may be controlled by the computer 822 in amanner known to those skilled in the art. Although the use of a rotatingelement may work well, it may limit the system 802. As should beappreciated, the use of rotating elements may be slow, and because thereare moving parts they tend to be less reliable.

In accordance with one embodiment, therefore, the polarizer 828 isconfigured to include a polarization modulator 836, such as photoelasticmodulator (PEM), in order to produce a fast and reliable spectroscopicellipsometer. The polarization modulator replaces the rotatingwaveplate. The polarization modulator 836 is an optical element thatperforms the same function as a rotating waveplate, but without thecostly speed and reliability problems. The polarization modulator 836allows electrical modulation of the phase of the light withoutmechanically rotating any optical components. Modulation frequencies ashigh as 100 kHz are readily attainable.

In an alternative embodiment, the analyzer 832 is configured to includea polarization modulator such as a PEM (Photoelastic Modulator) that canbe modulated electrically. In yet another embodiment, both the polarizerand analyzer contain polarization modulators, such as PEMs, that aremodulated at different frequencies.

Because the polarization modulator 836 can modulate at such a highfrequency, the polarization modulator 836 may be used to perform varioustechniques, which would otherwise be too slow. For example, thedifference between the polarized reflectivity of two structures may beobtained. To do this, a PEM may be combined with an acoustic opticalmodulator (AOM), where the AOM rapidly moves between the two structureswhile modulating the polarization state at a different (but related,such as multiple or submultiple) frequency. Signals at the sum and thedifference of the PEM and AOM modulation frequencies contain usefulinformation and can be detected with high signal-to-noise by synchronousdetection. Alternatively the AOM on the incident beam could be used incombination with a PEM in the analyzer.

Although not shown, the rotating waveplate may also be replaced by apolarization modulator in other types of scatterometric systems as forexample a polarization sensitive reflectometer.

Scatterometric Overlay Database

One aspect of the present invention provides a database ofscatterometric overlay information that may be utilized forscatterometic overlay determination.

In one implementation, one or more database are provided which includeone or more libraries of overlay information. The database informationis then used in overlay measurements.

In one implementation, the libraries are compiled using predeterminedtest patterns with artificially-induced overlay. Alternatively, thelibraries are produced using layer misregistrations programmed into thestepper. In another embodiment, the overlay that is induced orprogrammed has a progressive characteristic, varying within a particularrange.

The information stored into the database may include overlay dataregarding the actual overlay printed on the wafer, as induced via thetest pattern or by the stepper. Alternatively, this information isobtained from overlay actually measured on samples. The database mayfurther store scatterometry measurement records associated with theoverlay data. Such scatterometry measurement records may be obtained byperforming actual scatterometric measurements of the overlay data. Thedatabase may also include information regarding materials, processconditions, optical parameters, and other relevant data. The databaseinformation may be further enhanced by interpolation and otherpreprocessing.

The scatterometry database information may be utilized to improve theaccuracy and speed of overlay measurements by retrieving scatterometrydata associated with particular scatterometric measurements and processconditions recorded during actual measurements. Dynamic selection of ameasurement algorithm or methods may also be provided based on databaselookups. A further implementation utilizes the database to calibratescatterometric overlay measurement tools before or during productionline measurements.

Alternative Systems for Performing Scatterometry

According to an embodiment of the invention, acquisition of the spectraA through D (and of additional spectra if present) is performed using anoptical apparatus that may comprise any of the following or anycombination of the following apparatus: an imaging reflectometer, animaging spectroscopic reflectometer, a polarized spectroscopic imagingreflectometer, a scanning reflectometer system, a system with two ormore reflectometers capable of parallel data acquisition, a system withtwo or more spectroscopic reflectometers capable of parallel dataacquisition, a system with two or more polarized spectroscopicreflectometers capable of parallel data acquisition, a system with twoor more polarized spectroscopic reflectometers capable of serial dataacquisition without moving the wafer stage or moving any opticalelements or the reflectometer stage, imaging spectrometers, imagingsystem with wavelength filter, imaging system with long-pass wavelengthfilter, imaging system with short-pass wavelength filter, imaging systemwithout wavelength filter, interferometric imaging system (e.g. Linnikmicroscope, e.g. Linnik microscope as implemented in the KLA-Tencoroverlay measurements tools models 5100, 5200, 5300, Archer10, etc.available from KLA-Tencor of San Jose, Calif.), imaging ellipsometer,imaging spectroscopic ellipsometer, a scanning ellipsometer system, asystem with two or more ellipsometers capable of parallel dataacquisition, a system with two or more ellipsometers capable of serialdata acquisition without moving the wafer stage or moving any opticalelements or the ellipsometer stage, a Michelson interfereometer, aMach-Zehnder interferometer, or a Sagnac interferometer.

Several embodiments of an interferometer based imaging spectrometer, aswell as other types of imaging spectrometers such as filter based or the“push broom” approach, are described in U.S. Patent, having U.S. Pat.No. 5,835,214, issued 10 Nov. 1998, entitled “METHOD AND APPARATUS FORSPECTRAL ANALYSIS OF IMAGES”, by Cabib et al. System and Methodembodiments for film thickness mapping with spectral imaging aredescribed in U.S. Patent, having U.S. Pat. No. 5,856,871, issued 5 Jan.1999, entitled “FILM THICKNESS MAPPING USING INTERFEROMETRIC SPECTRALIMAGING”, by Cabib et al. An alternative architecture for spectralimaging based on LED illumination is described in U.S. Patent, havingU.S. Pat. No. 6,142,629, issued 7 Nov. 2000, entitled “SPECTRAL IMAGINGUSING ILLUMINATION OF PRESELECTED SPECTRAL CONTENT”, by Adel et al.These patents are incorporated herein by reference in their entirety forall purposes.

The imaging spectrometer or reflectometer used for acquisition of thespectra A through D from the four targets (and of additional spectra ifpresent) according to an embodiment of the invention may be of theFourier transform imaging spectrometer type as is well understood bythose skilled in the art. The imaging system of the Fourier transformimaging spectrometer should be capable of separating (resolving) thereflected or scattered light signals from the different targets (orsections of a compound scatterometry overlay target). Alternatively theimaging spectrometer or reflectometer used for acquisition ofscatterometry overlay signals may use a two-dimension detector where oneaxis contains the spatial information from the different scatterometryoverlay targets (or sections of a compound scatterometry overlay target)and the other detector axis contains spectrally resolved informationfrom light spectroscopically separated with a prism system ordiffraction grating system, for example or a system that is acombination of a prism and a grating. The illumination radiation may bewavelength selected prior to incidence on the target.

The spectra A through D obtained from the four targets (and additionalspectra if present) detected in the imaging spectrometers, imagingreflectometers, or any of the other optical systems identified above inconnection with various embodiments of the present invention may beunpolarized or selectively polarized. One or more of the unpolarizedlight or one or more of the polarization components of the reflected orscattered light from the targets may be detected with the imagingspectrometer or the imaging reflectometer.

In various implementations, separate detection systems may be used toseparately or simultaneously record one or more of the following lightsignals: unpolarized reflected light, polarized light with the electricfield substantially parallel to one major symmetry axis of one layer ofthe scatterometry overlay targets, polarized light with the electricfield substantially perpendicular to one major symmetry axis of onelayer of the scatterometry overlay targets, polarized light with theelectric field at an angle to one major symmetry axis of one layer ofthe scatterometry overlay targets, right-hand circularly polarizedradiation, left-hand circularly polarized radiation, and/or acombination of two or more of the previously listed polarization states.A separate detector system may be used to simultaneously record thesignal from part of the light source for the purposes of light noisemonitoring, and/or light level control, and/or light noise subtractionor normalization.

Various possible implementations of various embodiments of the presentinvention are illustrated in co-pending U.S. Provisional Application No.60/449,496, filed 22 Feb. 2003, entitled METHOD AND SYSTEM FORDETERMINING OVERLAY ERROR BASED ON SCATTEROMETRY SIGNALS ACQUIRED FROMMULTIPLE OVERLAY MEASUREMENT PATTERNS, by Walter D. Mieher et al. Thisprovisional application is herein incorporated by reference in itsentirety.

In one embodiment, each of the four targets is illuminated by radiationproduced by an optical system. The optical system may take the form of,among others, an optical source, a lensing system, a focusing system, abeam shaping system, and/or a directing system. In one embodiment, theradiation illuminating at least one of the targets is shaped as aradiation beam, with a relatively narrow beam cross section. In aparticular implementation, the beam is a laser beam. The radiationilluminating the targets interacts with structures comprised within thetargets and produces diffracted radiation components corresponding toeach target and denoted as S_(A), S_(B), S_(C), and S_(D). In oneembodiment, the illuminating beam is a broadband polarized beamcomprising a broad spectral range as is commonly used in spectroscopicellipsometry.

2. Scatterometry Overlay Technique Alternatives:

Several related techniques are described in the above related co-pendingU.S. Provisional Applications. These related techniques may be easilyintegrated with the techniques described herein.

In one embodiment of the invention, the targets (or compoundscatterometry target sections) with different programmed offsets +/−Fand +/−f0 as described above, or +/−F or other similar targetcombinations, are grouped together to enable simultaneous signalacquisition. In one implementation targets are arranged in a line toenable data acquisition while scanning the wafer or some or all of theoptics in one direction along the array of scatterometry overlaytargets. Arranging the targets in a linear array may also enable use ofan imaging spectrometer or reflectometer, where one detector axisseparates the signals from the different targets (or target sections)and the other detector axis detects the spectral information. In thiscase the imaging system images a linear or cylindrical image of thelinear target array into the prism or grating system. The imagingspectrometer or imaging reflectometer may contain an array of two ormore lenses (known to those skilled in the art as a lenslet array) toseparate and direct the reflected or scattered light from differenttargets or target sections.

In one embodiment, the primary offset F is optimized to provide largeror maximum sensitivity to overlay errors. For instance, an offset Fequal to ¼ of the pitch of the target provides high overlay sensitivitysince it is half-way in-between the two symmetry points where overlayerror sensitivity is minimum. The secondary offset f0 may be chosen suchthat the f0 is outside the region of interest for overlay measurements,such as equal to or beyond the specification limits, but it should notcause the uncertainty of the overlay measurement to allow the error thatan out-of-spec measurement can be interpreted as within specifications.Nevertheless, this is not a limitation on the range of f0. A large f0may decrease the accuracy of the overlay measurements for overlay errorsE between −f0 and +f0. For overlay errors E larger than |f0|, theaccuracy of the overlay measurement may be reduced due to extrapolationbeyond the region −f0 to +f0 and the accuracy of the linearapproximation may also be reduced.

Overlay measurements are most commonly done at or near the four cornersof the stepper field (sometimes with an additional measurement near thecenter of the field) in 5 to 25 fields per wafer in semiconductormanufacturing processes. For a system of four targets used to determineoverlay in the x direction and four targets used to determine overlay inthe y direction, according to an embodiment of the present invention, atotal of 8*4*5=160 measurements of scatterometry overlay targets may beused to determine the two dimensional overlay for a common overlaymeasurement sampling plan. More measurements may be conducted for moredetailed sampling plans.

According to another embodiment of the invention, a total of six targets(three for x and three for y, for example) can be used to determine twodimensional overlay for the sample. This may facilitate furthersimplification of the overlay metrology process, reduction in processingresources, and decrease of the time used in the metrology process. Inyet other implementations, additional targets or additional pairs oftargets may be produced on the sample and used in a substantiallysimilar manner with that described herein for determination of overlaybased on scatterometry, but adjusted for the increased number of targetsand corresponding number of diffracted radiation components. Themathematical methods for determination of the overlay error E can besimilarly adjusted to exploit the availability of increased informationprovided by such additional targets or additional pairs, including bypossibly accounting for higher order approximation terms in the formulafor the overlay error E.

Scatterometric Overlay Determination With Limited Refocusing

To improve the accuracy of scatterometry overlay determination, morethan one measurement is preferably carried out. One implementationutilizes a plurality of scatterometry overlay targets, and for eachtarget, the system makes one scatterometric measurement of overlay.Another implementation utilizes a single scatterometric target, or asingle scatterometric target area that comprises multiple targetsub-regions, and more than one scatterometric overlay measurement isperformed for that target or target area. In yet another embodiment, aplurality of targets or target regions are used, and more than onemeasurement is performed for some or all of the targets or targetregions.

Conventionally, the optical system is refocused for each individualmeasurement. This, however, can consume a lot of time thus decreasingthe processing speed of the system. For example, each focus sequence maytake between 0.1 and 1 seconds, and each wafer may include between 30 to70 sites with each site consisting of 8 targets. Using these numbers,refocusing may take up to as much as 560 seconds for each wafer.Considering there are typically 100s and 1000s of wafers to be inspectedthis number may be further increased to a completely unacceptable level.

In accordance with one embodiment of the present invention, therefore,multiple scatterometry overlay measurements are performed with limitedoptical refocusing in order to increase the processing speed andthroughput of the system. By limited optical refocusing, it is generallymeant that at least some new measurements are performed withoutrefocusing the optical system, i.e., multiple measurements are made withthe same focus setting. For instance, the optical system may beinitialized with a focus setting that is optimized for a plurality ofscatterometric measurements that will be performed, and no furtherrefocusing takes place during these individual scatterometricmeasurements. The optimized focus setting may be found once for theentire wafer, or it may be found periodically. When periodic, the focussetting may be established at preset increments of time duringinspection (e.g., every 30 seconds), for a particular location on thewafer (e.g., every 2×2 cm2 of wafer), for a particular characteristic ofthe target (e.g., similar line widths and spacing) and the like.

In one embodiment, the wafer includes a plurality of focus zones. Eachof the focus zones is initialized with a focus setting that is optimizedfor all of scatterometric measurements that will be performed within thefocus zone. Refocusing does not occur between individual scatterometricmeasurements inside the focus zone. As such, each target within thefocus zone is measured with the same optimized focus setting. Any numberof focus zones may be used.

The configuration of the focus zones may be widely varied. In oneimplementation, the focus zones correspond to a portion of the wafer. Byway of example, the wafer may be broken up into plurality of radialfocus zones emanating at the center of the wafer and working outwards,or into a plurality of angular focus zones, which separate the waferinto multiple quadrants. In another implementation, the focus zonescorrespond to a particular set of targets as for example, the targets atthe corners of each semiconductor device. In another implementation, thefocus zone corresponds to a particular target area that includes aplurality of targets (see for example 9A). In another implementation,the focus zones correspond to a particular target sub-region within thetarget areas (as for example the x or y directed group of targets shownin FIG. 9B). In yet another implementation, the focus zone correspondsto a particular sub region within the target itself.

A method of determining overlay will now be described. The methodgenerally includes optimizing the focus setting of a first zone. Themethod also includes performing a first set of measurements on aplurality of targets within the first zone. Each of the targets withinthe first zone is measured using the optimized focus setting of thefirst zone. That is, a first target is measured, and thereafter a secondtarget is measured without refocusing the optical system. Any number oftargets can be measured in this manner. The method further includesoptimizing the focus setting of a second zone. The method additionallyincludes performing a second set of measurements on a plurality oftargets within the second zone. Each of the targets within the secondzone is measured using the optimized focus setting of the second zone.That is, a first target is measured, and thereafter a second target ismeasured without refocusing the optical system. Any number of targetscan be measured in this manner.

In one example of this method, the first and second zones may representdifferent target areas that include a plurality of targets (See FIG.9A). In this example, each of the targets are located in close proximityto one another and therefore it can be assumed that variations in focusfrom one target to the next are minimal. The method generally includesoptimizing the focus setting in the target area, and thereaftermeasuring each of the targets in the target area with the optimizedfocus setting. For example, the first target is measured, thereafter theadjacent target is measured, and so on without ever refocusing theoptical system. When a first target area is measured, the system mayrepeat these steps on a second target area, as for example, a targetarea located at a different corner of the device.

In another example of this method, the first and second zones mayrepresent sub regions with a target area that includes a plurality oftargets. The sub regions may for example represent different targetorientations (See FIG. 9B). The method generally includes optimizing thefocus setting in the first sub regions (e.g., targets along the x axis),and thereafter measuring each of the targets in the sub region with theoptimized focus setting. For example, the first target is measured,thereafter the adjacent target is measured, and so on without everrefocusing the optical system. When the first sub region is measured,the method continues by optimizing the focus setting in the second subregions (e.g., targets along the y axis), and thereafter measuring eachof the targets in the sub region with the optimized focus setting. Forexample, the first target is measured, thereafter the adjacent target ismeasured, and so on without ever refocusing the optical system. Inanother example the system is refocused prior to the measurement on thefirst scatterometry overlay target in an xy scatterometry overlay targetgroup. After the scatterometry signal is measured for the first targetin the xy overlay target group, the rest of the targets may be measuredwithout refocusing. For example, an xy overlay target group comprisesfour scatterometry overlay targets for an overlay error determination inthe x direction and four scatterometry overlay targets for an overlayerror determination in the y direction.

Scatterometric Overlay Determination Using a Line Image

A system for scatterometric measurement of overlay using a line imagemay also be implemented.

In one embodiment, a scatterometric target is illuminated along a singleincidence line. The scattered radiation is intercepted by a dispersivesystem, such as a prism or diffraction grating. The scattered radiationis thereby dispersed as a function of wavelength. The dispersedradiation is then captured by a detector, such as a CCD camera. If thecamera is properly aligned, the radiation entering the field of view hasa two dimensional profile with points along the incidence linedistributed along the X-axis of the field of view, and variouswavelengths dispersed along the Y-axis. An example incidence line andfield of view are illustrated in FIG. 10.

The image captured by the camera can then be processed at pixel level todetermine overlay, possibly using the FT approach disclosed herein. Onceoverlay is measured along a particular incidence line, the wafer couldbe rotated by 90 degrees (or by any arbitrary angle) to measure overlayin a different direction. An advantage of the present invention is thatoverlay may be measured in more than one direction using a singleoptical system.

An alternative to illuminating a single incident line is illuminating alarger area but only capturing scattered radiation along a detectionline. The description provided above also applies to this embodiment,with appropriate modifications.

Algorithms

Various algorithms and methods may be employed for more efficiently andaccurately measuring overlay based on scatterometry.

Old methods of performing such calculations use a model-based method, ora differential method for calculating overlay. These conventionalmethods lack the accuracy that may be achieved by combining multiplealgorithms for refining and cross checking results. Also, these methodsdo not make good use of pre-existing information (like CD or profiledata).

In one general algorithm implementation, overlay is determined usingdata from a plurality of separate calculations of different productparameters.

In a first embodiment, a first calculation of overlay is performedaccording to a first technique (such as the differential method). Asecond calculation of overlay is then performed according to a secondtechnique (such as a model-based regression). The results are thencombined from the two calculations. The results may be combined invarious ways. For example, one calculation may be used to cross checkanother. Or one calculation may be used to provide initial values tospeed up the other calculation. Other combinations may also be used.

In a second embodiment, the speed and/or accuracy of an overlaymeasurement are enhanced by making use of other measured data. Forexample, film thickness data from the layers making up the target may befed into the algorithm. Such film thickness data could be measured usingan appropriate tool, such as an ellipsometer or reflectometer.Alternatively (or additionally), CD data could be provided from an SCDmeasurement (scatterometry critical dimension or scatterometry profilemeasurement) and used to speed up or improve the accuracy of thescatterometry calculations. Other data from a scatterometry profilemeasurement could be similarly used, such as height or three dimensionalprofile information. Other sources of CD data, like a CD SEM, could beused.

Combined Scatterometry and Imaging Targets

In an alternative implementation, the targets are designed for animaging based overlay metrology application, as well as for the abovedescribed scatterometry analysis. In other words, the scatterometry andimaging target structures are tightly integrated so that scatterometrymay be performed in conjunction with an image based overlay measurement.Preferably, the scatterometry target pairs are symmetrically positionedabout the center of the field of view. If symmetry is preserved in theillumination and collection channels of the imaging system, tool inducedshift and will be minimized. By example, Xa and Xa′ are twin (havesimilar magnitude but opposite sign offset) targets in the x direction.(Here Xa and Xa′ may correspond to the targets Xa and Xd in FIG. 1).Likewise, Xb and Xb′ are opposites. (Here Xb and Xb′ may correspond tothe targets Xb and Xc in FIG. 1). In the y direction, targets Ya and Ya′are opposites, while Yb and Yb′ are opposites.

FIG. 11 a is a top view representation of a first combination imagingand scatterometry target embodiment. In this example, the targetarrangement includes a set of four x direction targets for determiningoverlay using scatterometry and a set of four y direction targets fordetermining overlay using scatterometry. The targets are laid out sothat adjacent targets (with respect to the overlay measurementdirection) have an opposite offset.

In the illustrated example, target Xa has an opposite offset than targetXa′, and target Xb has an opposite offset than target Xb′. Likewise,targets Ya and Ya′ have opposite offsets, and targets Yb and Yb′ haveopposite offsets. In this example, the targets also include structureswhich can be used for imaged based overlay determination.

In the illustrated example, the target arrangement includes a blackborder structure 1104 on a first layer and a gray cross-shaped structure1102 on a second layer. Using image analysis methods, the center of theblack structure 1104 may then be compared with the center of the graystructure 1102 to determine an overlay error (if any).

Although this set of targets have an overall rectangular shape whichextends longer in the x direction than the y direction, of course, thetargets could have other shapes (e.g., square or any symmetricalpolygon) and/or extend longer in a direction other than x.

In other combinational target arrangements, the imaging structures arelaid out in the center of a symmetrically arranged set of scatterometrytargets. FIG. 11 b is a top view representation of a second combinationimaging and scatterometry target embodiment. As shown, scatterometrytargets are symmetrically arranged around a central image type target1152. In this example, the image type target 1152 is formed fromquadrants of line segments, where each quadrant is either in the x or ydirection. Suitable image type targets and techniques for determiningoverlay with same are described in the following U.S. patents andapplications: (1) U.S. Pat. No. 6,462,818, issued 8 Oct. 2002, entitled“OVERLAY ALIGNMENT MARK DESIGN” by Bareket, (2) U.S. Pat. No. 6,023,338,issued 8 Feb. 2000, entitled “OVERLAY ALIGNMENT MEASUREMENT OF WAFER”,by Bareket, (3) application Ser. No. 09/894,987, filed 27 Jun. 2001,entitled “OVERLAY MARKS, METHODS OF OVERLAY MARK DESIGN AND METHODS OFOVERLAY MEASUREMENTS”, by Ghinokver et al., and (4) U.S. Pat. No.6,486,954, issued 26 Nov. 2002, entitled “OVERLAY ALIGNMENT MEASUREMENTMARK” by Levy et al. These patents and applications are all incorporatedherein by reference in their entirety.

FIG. 11 c is a top view representation of a third combination imagingand scatterometry target embodiment. This target arrangement hasscatterometry target symmetrically arranged around a box-in-box typetarget 1154. A box-in-box target generally includes a first inner boxformed from a first layer surrounded by a second outer box structureformed in a second layer. The centers of the inner box structures may becompared to the center of the outer box structures to determine overlayerror (if present).

The above targets may be imaged in any suitable manner (e.g., asdescribed in the above referenced patents and applications by Bareket,Ghinovker et al., and Levy et al.) to determine overlay. The targetarrangements may also simultaneously or sequentially measured with anydesirable optical tool as described herein to determine overlay usingscatterometry techniques. In an alternative embodiment, thescatterometry targets may be simultaneously imaged along with theimaging type target structures. The resulting image may be subdividedinto the separate scatterometry targets and then the scatterometrytechniques applied to the image signals for each target (e.g.,intensity). The image may be obtained at the same time as, or before orafter the scatterometry overlay measurements. The imaging system may bea high-resolution microscope such as the system in the KLA-Tencor 5300or Archer overlay measurement systems available from KLA-Tencor of SanJose, Calif. Alternatively, the imaging system may be a lower resolutionimaging system used for other purposes that may include wafer alignmentor pattern recognition.

Mask Alignment During Imprint Lithography

Because the mask and sample are typically in close proximity (separatedby the fluid to be polymerized) during nano-imprint lithography, thepatterned surface of the mask, the fluid, and the patterned sample to bealigned to can be considered to be functionally equivalent to ascatterometry overlay target. All of the methods, techniques and targetsdefined for scatterometry overly would then be applicable to alignmentprocedures.

In one embodiment, the measurement instrument would project radiation(preferably light) through the mask and onto an area of the mask andwafer which contains one or more scatterometry overlay targets. Thechange in properties of the reflected light due to scattering ordiffraction may then be used to determine the offset between the patternon the mask and the pattern on the wafer. The wafer is then movedrelative to the mask (or vice versa) to achieve the desired offset. Amore accurate alignment may the be achieved, rather than withconventional alignment techniques such as direct imaging or moirétechniques. The instrument could be a reflectometer, polarizedreflectometer, spectrometer, imaging reflectometer, imaginginterferometer, or other such instrument as described herein or in theabove referenced provisional applications.

Disposition of Scatterometry Overlay Targets

The accuracy of scatterometry overlay systems can be improved by takingmeasurements at multiple targets located across the surface of interest.In one implementation, the scatterometry overlay system may utilize aplurality of scatterometry targets at various locations across thesurface of interest and for each target the system may make onescatterometric measurement of overlay. In another implementation, thescatterometry overlay system may utilize a plurality of scatterometrytarget areas at various locations across the surface of interest. Thescatterometry target areas comprise multiple targets, each of which canbe measured by the scatterometry overlay system. By way of example, thescatterometry targets or scatterometric target areas may be located atthe corners of one or more devices being formed on a wafer. In addition,the scatterometry targets may generally include a grating structure,which is measurable by the scatterometry overlay system.

The number of targets generally depends on the available space on thesurface of interest. In most cases, the targets are placed in the scribeline between devices on a wafer. The scribe line is the place on thewafer where the wafer is separated into dies via sawing or dicing andthus the circuit itself is not patterned there. In cases such as this,the number of targets may be limited, at least in part, by thenarrowness of the scribeline. As should be appreciated, the scribe linestend to be narrow so as to maximize the amount of devices on the wafer.

In accordance with one embodiment of the present invention, the targetsare strategically placed on the surface of interest in order to overcomeany space constraints while increasing the number of targets. In oneimplementation, at least two targets are placed substantiallycollinearly in a first direction. For example, they may be placedcollinearly in the x-direction or the y-direction. This arrangement maybe useful when confronted with narrow spaces as for example scribelines. In another implementation, multiple targets are disposedcollinearly in multiple directions. For example, multiple targets may bedisposed collinearly in both the x direction and the y-direction. Thisarrangement may be useful at the corner of a device as for example atthe intersection of two scribe lines.

Although the examples given are directed at a Cartesian coordinatesystem as defined on the surface of interest, it should be noted thatthe coordinate system may be oriented arbitrarily on the surface orinterest with the x and y axis being rotated or possibly interchanged.Alternatively or in combination with the Cartesian coordinate system,any other coordinate system may be used such as for example, a polarcoordinate system.

FIG. 9A is a top view diagram of a scatterometric target area 900 havingone or more targets 902, in accordance with one embodiment of thepresent invention. The scatterometric targets 902 are generally providedto determine the relative shift between two or more successive layers ofa substrate or between two or more separately generated patterns on asingle layer of a substrate. By way of example, the scatterometrictargets may be used to determine how accurately a first layer alignswith respect to a second layer disposed above or below it or howaccurately a first pattern aligns relative to a preceding or succeedingsecond pattern disposed on the same layer.

As shown in FIG. 9A, the scatterometric target area 900 includes atleast two substantially collinear targets 902. By collinear, it isgenerally meant that the centers of symmetry for each of the targets 902lie on the same axis 904. By way of example, the axis 904 may be alignedwith a conventional coordinate system (Cartesian, polar, etc.) or somevariation thereof. By placing the targets 902 collinearly, thescatterometric target area 900 does not take up as much width W andtherefore may be placed in constrained places as for example in thescribeline of the wafer.

The targets 902 are generally juxtaposed relative to one another alongthe axis 904. In most cases, the juxtaposed targets 902 are spatiallyseparated from one another so that they do not overlap portions of anadjacent target 902. Each of the targets 902 is therefore distinct,i.e., represents a different area on the substrate. This is typicallydone to ensure that each of the targets 902 is properly measured. Thespace 906 between targets 902 produces distortions in the optical signaland therefore it is excluded from the overlay calculation. The size ofthe space 906 is typically balanced with the size of the targets 902 soas to provide as much information as possible for the measurement ofoverlay. That is, it is generally desired to have larger targets 902 andsmaller spaces 906 there between. The space 906 between targets 902 maybe referred to as an exclusion zone.

The targets 902 may be widely varied, and may generally correspond toany of those overlay targets that can be measured via scatterometry. Byway of example, the targets 902 may generally include one or moregrating structures 908 having parallel segmented lines 910. Although nota requirement, the segmented lines 910 for the collinear targets 902 aregenerally positioned in the same direction, which may be parallel ortransverse to the axis 904. In most cases, some of the segmented lines910 are perpendicular to the axis 904 and some are parallel to the axis904 to enable overlay measurements in x and y. Furthermore, the targets902 may have an identical configuration or they may have a differentconfiguration. Configuration may for example include the overall shapeand size of the target 902 or perhaps the line width and spacing of thesegmented lines 910 associated with the grating structure 908 containedwithin the target 902. Preferably the targets used for the overlaymeasurement in a particular direction, for example x direction, aredesigned to have the same configuration except for the programmed ordesigned overlay offsets.

The number of targets may also be widely varied. As should beappreciated, increasing the number of targets, increases the number ofdata collection points and therefore the accuracy of the measurement.The number of targets 902 generally depends on the overall size of thetargets 902 and the space constraints in the direction of the axis 904.In the illustrated embodiment, eight side by side targets 902 arepositioned within the scatterometric target area 900. A scatterometrictarget area may be equivalent to an x-y scatterometry overlay targetgroup as discussed above.

Using the above mentioned targets 902, scatterometric overlaymeasurements may be made sequentially, one target at a time, to measureoverlay while eliminating effects due to variations in other sampleparameters, such as film thickness. This can be accomplished viacontinuously scanning of the scatterometric target area (including forexample the targets and the spaces there between) or by stepping to eachof the targets. Alternatively, measurements may take place substantiallysimultaneously using two or more scatterometry signal beams for two,more than two, or all targets to increase throughput. The multiplescatterometry signal beams may come from more than one substantiallyindependent scatterometry optical systems, or they may share much of theoptical system, for example they may share the same light source, thesame beam directing optics, or the same detector system.

Although the method described above includes placing the centers ofsymmetry for each of the targets substantially collinear, it should benoted that the centers of symmetry may be offset from the axis so longas a measurable portion of the targets still falls on the same axis.

Furthermore, although the method described above includes placingtargets of similar orientation along the same axis, it should be notedthat some of the targets may be positioned with a different orientation.For example, a first group of the targets 902 may have segmented linespositioned in the x dimension while a second group of the targets 902may have segmented lines position in the y dimension.

Moreover, although the targets 902 are only shown positioned along asingle axis 904, it should be noted that the targets may be positionedon multiple axis. For example, as shown in FIG. 9B, a first group oftargets 902A may be disposed collinearly along a first axis 904A and asecond group of targets 902B may be disposed collinearly along a secondaxis 904B. This implementation permits independent measurement ofoverlay in at least two directions. The first and second axis aretypically transverse to one another and more particularly perpendicularone another. In the illustrated embodiment, the first axis 904Acorresponds to the X-dimension while the second axis 904B correspondsthe Y-dimension. Furthermore, each group consists of four targets 902.This implementation permits independent measurement of overlay in the Xand Y directions.

Further still, although the targets have been described as havingfeatures (e.g., segmented lines) in substantially one direction, itshould be noted that the targets may include features in more than onedirection. In one implementation, for example, one or more of thecollinearly positioned targets include features that permitscatterometric overlay measurement in first and second directions. Byway of example, the features such as the segmented lines may bepositioned in both the X and Y dimensions. In this case, the need fordisposing targets along more than one axis as shown in FIG. 9B isreduced or eliminated. That is, if each target has features that permittwo-dimensional scatterometric measurements, overlay may be determinedalong both the X- and Y-axes using a single set of targets disposedsubstantially-collinearly along a single axis. Alternatively, one ormore targets may include one or more sub-targets. If the sub-targetshave features that permit two-dimensional scatterometric measurements,the number of targets desirable for a particular degree of measurementaccuracy may be reduced, and the targets may be disposed along a singleline.

Additionally, targets disposed along one or more axis may be used formeasurement of more then one parameter. For example, a first set oftargets may be used for scatterometric measurement of wavelength alongthe X-axis and a second set of targets may be used for scatterometricmeasurement of spatial resolution along the Y-axis. In an alternativeimplementation, scatterometric measurement of spatial resolution may beperformed along the X-axis while spectral measurements are performedalong the Y-axis.

Combined CD and Overlay Marks

Scatterometry measurement targets consume a significant area of thewafer for both metrology of CD and overlay. This wafer area becomes veryvaluable as design rule shrinks. Currently, scatterometry overlay marksmay consume >35×70 um space for each xy scatterometry overlay targetgroup or mark on the wafer. These are used only for overlay measurementsand therefore the manufacturers consider the loss of wafer space asundesirable. Therefore, it is desirable to reduce the total wafer arearequired for measurement targets or measurement features. Changes tooptical system design to enable measurements on smaller targets mayresult in greater complexity of the optical system and potentiallycompromise measurement performance. In scatterometry overlay measurementas describe herein, the target area typically consists of four gratingsfor each axis (X and Y). Each of these gratings is typically larger than15×15 um with a limited opportunity to shrink it further usingconventional techniques. Each grating is composed of a first layergrating (e.g. STI) and a top layer grating (e.g. gate.). Each of the twolayers has a programmed offset, which is typically smaller than thepitch of the top grating. In many cases the top layer is photoresist. Anoverlay measurement is achieved by analyzing the spectra of a reflectedlight from each of these gratings.

In scatterometry critical dimension (CD) measurement, the target areatypically consists of a single grating, which may be positioned alongeither axis (X or Y). In some cases, the target area may includemultiple gratings for each axis (X and Y). Each of these gratings istypically about 50×50 um. The measurement is typically performed on asingle process layer target with no pattern underneath. This measurementis typically done on a photoresist pattern following a resistdevelopment step in a lithographic patterning process or following anetch or CMP process in other modules of the fabrication. A CDmeasurement is achieved by analyzing the spectra of a reflected lightfrom the grating(s) as described in the above referenced U.S. Pat. No.6,590,656 by Xu, et al.

In accordance with one embodiment of the present invention, thescatterometry CD marks and the scatterometry overlay marks are combinedto enable the fab to save wafer space and to print larger scatterometryoverlay marks with no impact on the wafer scribeline. The combined markis constructed with a scatterometry CD target as the first layer andscatterometry overlay target patterns as the top layer. This results inzero or minimal additional scribe line space allocated to scatterometryoverlay.

FIG. 12 is a diagram of a combined mark 1200, in accordance with oneembodiment of the present invention. The combined mark 1200 provides forboth scatterometry CD measurement and scatterometry overlay measurementat different steps in the wafer manufacturing process. The combined mark1200 is formed on at least two layers of the wafer, particularly a firstlayer L1 and a top layer L2. The first layer L1 includes scatterometryCD/profile targets 1202 and the top layer L2 includes scatterometryoverlay targets 1204. Although shown as separate layers in the diagram,it should be noted that the scatterometry overlay targets 1204 are builton (over) the scatterometry CD profile targets 1202. The scatterometryCD/profile targets 1202 form an L1 scatterometry CD mark, which can bemeasured to determine CD after formation or processing of the L1pattern. The scatterometry overlay targets 1204 cooperate with thescatterometry CD/profile targets 1202 to form an L2-L1 scatterometryoverlay mark, which can be measured to determine overlay between thelayers after formation of the L2 pattern (which comes after L1 patternformation). As should be readily apparent, this method may be repeatedto produce a layer 2 L2 scatterometry CD/profile target(s) followed by aLayer 3 L3 pattern to create an L3-L2 scatterometry overlay mark ortarget area.

The configuration of the scatterometry CD/profile targets 1202 andscatterometry overlay targets 1204 may be widely varied. In theillustrated embodiment, the scatterometry CD/profile targets 1202disposed on L1 include a first grating 1206 oriented in a firstdirection and a second grating 1208 oriented in a second direction. Thefirst direction may be orthogonal to the second direction. By way ofexample, the first grating 1206 may include vertical lines while thesecond grating 1208 may include horizontal lines. In addition, thescatterometry overlay targets 1204 disposed on L2 include a first groupof gratings 1210 and a second group of gratings 1212. Both the first andsecond groups of gratings 1210, 1212 include one or more gratings 1214.The number of gratings 1214 may be widely varied. In one implementation,both the first and second groups 1210 and 1212 include four gratings1214. The gratings 1214A in the first group 1210 are oriented in thefirst direction, and the gratings 1214B in the second group 1212 areoriented in the second direction. By way of example, the gratings 1214Ain the first group 1210 may include vertical lines while the gratings1214B in the second group 1212 may include horizontal lines.

In order to produce an L2-L1 overlay mark, the first group of gratings1210 is positioned over the first grating 1206 of the CD/profile targets1202, and the second group of gratings 1212 is positioned over thesecond grating 1208 of the CD/Profile targets 1202. This places gratingswith similarly directed lines together, i.e., vertical lines withvertical lines and horizontal lines with horizontal lines. The firstgroup of gratings 1210 cooperates with the first grating 1206 of theCD/profile targets 1202 and the second group of gratings 1212 cooperateswith the second grating 1208 of the CD/profile targets 1202. Thealignment between layers is determined by the shift produced between thecorresponding lines of these cooperating structures. The vertical lines,for example, may be used to determine X overlay and the horizontallines, for example, may be used to determine Y overlay.

Although the first and second gratings 1206 and 1208 of the CD mark areshown together, it should be noted that they may be placed apart. Whenimplemented apart, the first group of gratings 1210 and the second groupof gratings 1212 would also be placed apart, i.e., the first group ofgratings 1210 goes with the first grating 1206 and the second group ofgratings 1212 goes with the second grating 1208.

The advantages of combining overlay and CD marks are numerous. Differentembodiments or implementations may have one or more of the followingadvantages. One advantage of combining marks is in the ability to reducethe need for additional wafer space for scatterometry overlay targets.Another advantage is that larger scatterometry overlay targets may beallowed if they do not require as much additional scribe line space.Larger scatterometry overlay targets may make optical design or opticsmanufacturing easier and may provide better scatterometry overlaymetrology performance than on smaller scatterometry overlay targets.

Combination of Scatterometry Overlay and CDSEM

The purpose of this embodiment is to enable measurement of criticaldimensions on a semiconductor wafer with an electronic microscope(CD-SEM) and measurement of overlay using scatterometry on the samemeasurement system or using linked measurement systems sharing at leastpart of a robotic wafer handling system. The established methods ofmeasuring critical dimensions and overlay commonly require schedulingand operating separate measurement systems. One disadvantage of theestablished methods of measuring critical dimensions and overlay onseparate measurement systems is the additional time required to scheduleand run separate operations on separate metrology tools. Anotherdisadvantage is the redundancy of common parts and the costs associatedtherewith.

In order to overcome these disadvantages, a metrology system thatcombines Scatterometry Overlay and CDSEM may be provided. In oneembodiment, a scatterometry overlay measurement (SCOL) system isintegrated with a CDSEM system such that the CDSEM and SCOL systemsshare at least part of the robotic wafer handling system and/or datasystems. Alternatively, the CDSEM and the scatterometry overlay systemsmay be separate systems capable of independent operation, but linked insuch a way that they share at least part of a robotic wafer handlingsystem.

In operation, a wafer, a group of wafers, or batch of multiple wafersmay be introduced to the combined metrology system by loading the wafercontainer onto the robotic wafer handling system dedicated to thiscombined metrology system. Measurement recipes may be selectedspecifying CDSEM measurements on some or all of the wafers andscatterometry overlay measurements on some or all of the wafers. TheCDSEM measurements and the SCOL measurements may be specified togetherin one or more recipes, or may be specified in separate recipes. TheCDSEM and SCOL measurements may be done on the same wafers or ondifferent wafers or on some of the same wafer and some different wafers.The CDSEM and SCOL systems may operate in parallel, or in series.

One example of the combined metrology system would be integration of ascatterometry system capable of scatterometry overlay measurements (suchas a spectroscopic ellipsometer, spectroscopic polarized reflectometer,or +/−1 order diffraction scatterometer) inside a CD-SEM such as any ofthose manufactured by KLA-Tencor of San Jose, Calif. Another example ofa combined metrology system would be a linked system comprising ascatterometry overlay system, a CDSEM such as any of those manufacturedby KLA-Tencor of San Jose, Calif., a robotic handler, and a waferscheduling system. Communication to factory automation and/or factoryinformation, and/or factory process control systems may be throughseparate communication or automation systems or may be at leastpartially or completed shared.

One advantage of the combined CDSEM and SCOL metrology system is thereduction in overall time required to complete scheduling and/orperforming the CDSEM and scatterometry overlay measurements. At leastone queue delay time may be eliminated. Performing CDSEM and overlaymeasurements in parallel can save at least part of the time required forseparate measurement operations.

FIGS. 13A-13D show variations of a combined metrology tool 1300, inaccordance with several embodiment of the present invention. In all thefigures, the combined metrology tool 1300 includes a robotic waferhandling system 1302, a critical dimensioning scanning electronmicroscope (CD-SEM) 1304, a scatterometry overlay (SCOL) measurementinstrument 1306, a wafer load position A 1308 and a wafer load positionB and 1310, respectively. The robotic wafer handling system 1302 isconfigured to transfer wafers to and from the CD-SEM 1304 and SCOLmeasurement instrument 1306 as well as to and from the wafer loadpositions A and B 1308 and 1310. The critical dimensioning scanningmicroscope 1304 is configured to measure the critical dimensions thatmay include, for example, linewidth, top linewidth, via diameter,sidewall angle and profile. The scatterometry overlay measurementinstrument 1306 is configured to measure the overlay as for examplebetween two layers disposed on the wafer. The wafer load position A andwafer load position B are configured to hold one or more wafers. In mostcases, they hold a plurality of wafers. The wafers may be from the samelot of from a different lot.

In FIGS. 13A and D, the CD-SEM 1304 and the SCOL measurement instrument1306 are separate systems that are integrated via the robotic waferhandling system 1302. In FIG. 13B, the SCOL measurement instrument 1306is integrated into the CDSEM 1304. In FIG. 13C, the SCOL measurementinstrument 1306 is integrated into the robotic wafer handling system1302.

In one operation, some of the wafers from wafer load position A and/or Bhave critical dimensions measured at the CD-SEM and thereafter haveoverlay measured at the scatterometry overlay measurement instrument.The wafer can be measured by both processes without being removed fromthe system, i.e., the wafer handling as well as the throughput issuesassociated therewith are reduced. In another operation, some wafers fromwafer load position A and/or B have critical dimensions measured at theCDSEM and some other wafers from wafer load position A and/or B haveoverlay measured on SCOL measurement instrument. In any of theseoperations, the CDSEM and SCOL measurement instrument can proceedindependently and simultaneously.

FIG. 14 is a flow diagram 1400 using a combined metrology tool, inaccordance with one embodiment of the present invention. The methodgenerally includes step 1402 where a group of wafers are received by themetrology tool. By way of example, the wafers may be a wafer lot that isloaded at position A in FIG. 13. Following step 1402, the process flow1400 proceeds to step 1404 where the critical dimensions of a wafer fromthe group of wafers is measured. By way of example, the criticaldimension measurements may be performed by a CDSEM as for example theCDSEM shown in FIG. 13. The process flow 1400 also proceeds to step 1406where the overlay of a wafer from the group of wafers is performed by aSCOL measurement instrument as for example the instrument shown in FIG.13. Steps 1404 and 1406 may be performed at the same time on differentwafers. Steps 1404 and 1406 may be performed on the same wafer in asequence of operations, as for example, from CD to overlay or fromoverlay to CD. The transferring of the wafer may for example beperformed by the robotic system shown in FIG. 13. When all themeasurements are performed, the process flow proceeds to step 1408 wherethe group of wafers are released from the metrology tool.

Combination of Scatterometry Overlay and Other Metrology or InspectionMethods

Scatterometry overlay may be combined with scatterometry profile orscatterometry critical dimension systems, or other semiconductormetrology or inspections systems. Scatterometry overlay may beintegrated with a semiconductor process tool, for example a lithographyresist process tool (also known as a resist track). Integration ofmetrology systems with process systems and combinations of metrologysystems are described in (1) U.S. patent application, having U.S. patentSer. No. 09/849,622, filed 4 May 2001, entitled “METHOD AND SYSTEMS FORLITHOGRAPHY PROCESS CONTROL”, by Lakkapragada, Suresh, et al. and (2)U.S. patent, having U.S. Pat. No. 6,633,831, issued 14 Oct. 2003,entitled “METHODS AND SYSTEMS FOR DETERMINING CRITICAL DIMENSION AND ATHIN FILM CHARACTERISTIC OF A SPECIMAN”, by Nikoonahad et al, whichapplications are incorporated herein by reference in their entirety.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. Therefore, the described embodiments should be taken asillustrative and not restrictive, and the invention should not belimited to the details given herein but should be defined by thefollowing claims and their full scope of equivalents.

1. A method for determining overlay between a plurality of firststructures in a first layer of a sample and a plurality of secondstructures in a second layer of the sample, the method comprising:providing targets A, B, C and D that each include a portion of the firstand second structures, wherein the target A is designed to have anoffset Xa between its first and second structures' portions, wherein thetarget B is designed to have an offset Xb between its first and secondstructures' portions, wherein the target C is designed to have an offsetXc between its first and second structures' portions, wherein the targetD is designed to have an offset Xd between its first and secondstructures' portions, wherein each of the offsets Xa, Xb, Xc and Xd isdifferent from zero, Xa is an opposite sign and differs from Xb, and Xcis an opposite sign and differs from Xd; illuminating the targets A, B,C and D with electromagnetic radiation to obtain spectra S_(A), S_(B),S_(C), and S_(D) from targets A, B, C, and D, respectively; anddetermining and storing any overlay error between the first structuresand the second structures using a scatterometry technique based on theobtained spectra S_(A), S_(B), S_(C), and S_(D), wherein obtaining thespectra S_(A), S_(B), S_(C), and S_(D) comprises acquiring radiationfrom the targets A, B, C, and D using an optical apparatus comprising(i) a spectroscopic normal incidence polarized differentialreflectometer and an oblique incidence spectroscopic ellipsometer or(ii) a spectroscopic near-normal incidence polarized differentialreflectometer and an oblique incidence spectroscopic ellipsometer. 2.The method of claim 1, wherein determining any overlay error comprises:determining a difference spectrum D1 from the spectra S_(A) and S_(B);determining a difference spectrum D2 from the spectra S_(C) and S_(D);determining any overlay error by performing a linear approximation basedon the difference spectra D1 and D2.
 3. The method of claim 1, whereineach of the targets A, B, C, and D comprises a grating structure Ga1having periodic structures with a period Ta1 disposed at least partiallywithin the first layer and a grating structure Ga2 having periodicstructures with a period Ta2 disposed at least partially within thesecond layer, wherein the first period Ta1 and the second period Ta2 aresubstantially identical, and wherein the offsets Xa, Xb, Xc, and Xd areeach produced by offsetting the structures with the period Ta1 of thegrating structure Ga1 with respect to the structures with the period Ta2of the grating structure Ga2 by the sum of a first distance F and asecond distance f0, wherein the second distance f0 has a smallerabsolute value than the first distance F.
 4. The method of claim 1,wherein the targets A, B, C and D are disposed along a substantiallystraight line.
 5. The method of claim 4, wherein the target B isdisposed between the target A and the target C, and the target C isdisposed between the target B and the target D.
 6. The method of claim1, the method further comprising: producing an additional target E, theadditional target E including a portion of the first and secondstructures with an offset Y there between; illuminating the additionaltarget E with electromagnetic radiation to obtain spectra S_(E); andwherein the determining any overlay error is further based on thespectrum S_(E).
 7. The method of claim 1, wherein the optical apparatusincludes a spectroscopic normal incidence polarized differentialreflectometer and an oblique incidence spectroscopic ellipsometer. 8.The method of claim 1, wherein the optical apparatus includes aspectroscopic near-normal incidence polarized differential reflectometerand an oblique incidence spectroscopic ellipsometer.
 9. The method ofclaim 1, wherein at least one of the spectra S_(A), S_(B), S_(C), andS_(D) comprises electromagnetic radiation that is unpolarized orselectively polarized or selectively analyzed.
 10. The method of claim1, wherein at least one of the spectra S_(A), S_(B), S_(C), and S_(D)comprises electromagnetic radiation that is unpolarized reflected light,polarized light with the electric field substantially parallel to asymmetry axis of at least one set of structures of at least one of thetargets A, B, C or D, polarized light with the electric fieldsubstantially perpendicular to a symmetry axis of at least one set ofstructures of at least one of the targets A, B, C or D, polarized lightwith the electric field at an angle with respect to a symmetry axis ofat least one set of structures of at least one of the targets A, B, C orD, right-hand circularly polarized radiation, or left-hand circularlypolarized radiation.
 11. The method of claim 1, wherein the targets A,B, C and D are disposed in a two dimensional configuration.
 12. Themethod of claim 11, wherein the targets A and B are disposed along afirst axis, the targets C and D are disposed along a second axis, andthe first axis and the second axis are substantially parallel.
 13. Themethod as recited in claim 2, wherein the linear approximation is basedon a property P1 of the difference spectrum D1 and a property P2 of thedifference spectrum D2.
 14. The method of claim 13, wherein theproperties P1 and P2 of the difference spectra D1 and D2 each areselected from a group consisting of light noise, stability, drift,spectral characteristics, and light level.
 15. The method of claim 13where determining the properties P1 and P2 comprises obtaining orprocessing one or more of radiation characteristics of the differencespectra D1 and D2, respectively, selected from a group consisting ofintensity, spectral intensity of diffracted radiation, R(lambda) ofdifferent radiation, spectral intensity of transverse electric fieldpolarization R(Te, lambda), spectral intensity of transverse magneticfield polarization R(Tm, lambda), spectral intensity of S-polarizationreflectivity Rs (lambda), spectral intensity of P-polarization,reflectivity Rp(lambda), optical phase, wavelength, diffraction angle,spectroscopic ellipsometry parameters, alpha, beta, cos(delta), andtan(psi).
 16. A method for determining overlay error between a pluralityof first structures in a first layer of a sample and a plurality ofsecond structures in a second layer of the sample, the methodcomprising: providing targets A, B, C, and D that each include a portionof the first and second structures, wherein the target A is designed tohave an offset Xa between its first and second structures' portions,wherein the target B is designed to have an offset Xb between its firstand second structures' portions, wherein the target C is designed tohave an offset Xc between its first and second structures' portions,wherein the target D is designed to have an offset Xd between its firstand second structures' portions, wherein each of the offsets Xa, Xb, Xc,and Xd is different from zero, Xa is an opposite sign and differs fromXb, and Xc is an opposite sign and differs from Xd; illuminating thetargets A, B, C, and D with electromagnetic radiation to obtain spectraS_(A), S_(B), S_(C), and S_(D) from targets A, B, C, and D,respectively; and determining and storing any overlay error between thefirst structures and the second structures using a scatterometrytechnique based on the obtained spectra S_(A), S_(B), S_(C), and S_(D),and without using calibration or modeling data to determine any overlayerror, wherein obtaining the spectra S_(A), S_(B), S_(C), and S_(D)comprises acquiring radiation from the targets A, B, C, and D using animaging spectroscopic ellipsometer, and wherein an illumination andimaging NA's of the imaging spectroscopic ellipsometer are chosen tooptimize the performance of the instrument on scattering structures byensuring that only the zeroth diffraction order is collected.
 17. Asystem for determining overlay between a plurality of first structuresin a first layer of a sample and a plurality of second structures in asecond layer of the sample, comprising: a scatterometry module forilluminating plurality of targets A, B, C and D with electromagneticradiation to obtain spectra S_(A), S_(B), S_(C), and S_(D) from thetargets A, B, C, and D, respectively; and a processor operable fordetermining any overlay error between the first structures and thesecond structures using a scatterometry technique based on the obtainedspectra S_(A), S_(B), S_(C), and S_(D), wherein the targets A, B, C andD each include a portion of the first and second structures, wherein thetarget A is designed to have an offset Xa between its first and secondstructures' portions, wherein the target B is designed to have an offsetXb between its first and second structures' portions, wherein the targetC is designed to have an offset Xc between its first and secondstructures' portions, wherein the target D is designed to have an offsetXd between its first and second structures' portions, wherein each ofthe offsets Xa, Xb, Xc and Xd is different from zero, Xa is an oppositesign and differs from Xb, and Xc is an opposite sign and differs fromXd, wherein the scatterometry module is an optical apparatus in the formof (i) a spectroscopic normal incidence polarized differentialreflectometer and an oblique incidence spectroscopic ellipsometer or(ii) a spectroscopic near-normal incidence polarized differentialreflectometer and an oblique incidence spectroscopic ellipsometer. 18.The system of claim 17, wherein determining any overlay error comprises:determining a difference spectrum D1 from the spectra S_(A) and S_(B);determining a difference spectrum D2 from the spectra S_(C) and S_(D);determining any overlay error by performing a linear approximation basedon the difference spectra D1 and D2.
 19. The system of claim 17, whereinthe targets A, B, C and D are disposed along a substantially straightline.
 20. The system of claim 17, wherein the targets A, B, C and D aredisposed in a two dimensional configuration.
 21. The system of claim 17,wherein the processor is further operable for: producing an additionaltarget E, the additional target E including a portion of the first andsecond structures with an offset Y there between; illuminating theadditional target E with electromagnetic radiation to obtain spectraS_(E); and wherein the determining any overlay error is further based onthe spectrum S_(E).
 22. The system of claim 17, wherein the opticalapparatus is a system comprising a spectroscopic normal incidencepolarized differential reflectometer and an oblique incidencespectroscopic ellipsometer.
 23. The system of claim 17, wherein theoptical apparatus is a system comprising a spectroscopic near-normalincidence polarized differential reflectometer and an oblique incidencespectroscopic ellipsometer.
 24. The system of claim 17, wherein at leastone of the spectra S_(A), S_(B), S_(C), and S_(D) compriseselectromagnetic radiation that is unpolarized or selectively polarizedor selectively analyzed.
 25. The system of claim 17, wherein at leastone of the spectra S_(A), S_(B), S_(C), and S_(D) compriseselectromagnetic radiation that is unpolarized reflected light, polarizedlight with the electric field substantially parallel to a symmetry axisof at least one set of structures of at least one of the targets A, B, Cor D, polarized light with the electric field substantiallyperpendicular to a symmetry axis of at least one set of structures of atleast one of the targets A, B, C or D, polarized light with the electricfield at an angle with respect to a symmetry axis of at least one set ofstructures of at least one of the targets A, B, C or D, right-handcircularly polarized radiation, or left-hand circularly polarizedradiation.
 26. The system as recited in claim 18, wherein the linearapproximation is based on a property P1 of the difference spectrum D1and a property P2 of the difference spectrum D2.
 27. The system of claim26, wherein the properties P1 and P2 of the difference spectra D1 and D2each are selected from a group consisting of light noise, stability,drift, spectral characteristics, and light level.
 28. The system ofclaim 26, where determining the properties P1 and P2 comprises obtainingor processing one or more of radiation characteristics of the differencespectra D1 and D2, respectively, selected from a group consisting ofintensity, spectral intensity of diffracted radiation, R(lambda) ofdifferent radiation, spectral intensity of transverse electric fieldpolarization R(Te, lambda), spectral intensity of transverse magneticfield polarization R(Tm, lambda), spectral intensity of S-polarizationreflectivity Rs (lambda), spectral intensity of P-polarization,reflectivity Rp(lambda), optical phase, wavelength, diffraction angle,spectroscopic ellipsometry parameters, alpha, beta, cos(delta), andtan(psi).
 29. The system of claim 19, wherein the target B is disposedbetween the target A and the target C, and the target C is disposedbetween the target B and the target D.
 30. The system of claim 20,wherein the targets A and B are disposed along a first axis, the targetsC and D are disposed along a second axis, and the first axis and thesecond axis are substantially parallel.